The Industrial Age initiates a completely different phase of the game, as railroads allow units to move from one side of your territory to the other in a flash. The introduction of nationalism that allows for drafting of units and the increased size of cities (13+) that grants an extra bonus to defenders makes this a more difficult era for capturing territory. The units that exist during the Industrial and Modern Ages fight under different circumstances and in a different environment than earlier ones, and that should be taken into account when building them. As a result, they deserve a separate page from the other units.
Rifleman (4/6/1): Love them or hate them, these units have a big effect on the way the game is played. Rifles are easy to build after factories come along and can be drafted in a pinch, with the overall effect that they make cities much more different to capture. Although the AI will use these units as attckers, I don't suggest that you do except for in a critical situation. One of the most important things about riflemen is that they don't require any resources to build, so everyone and everyone will have these guys. Expect to gobs of rifles everywhere as soon as the AI civs get the tech Nationalism. If you put a couple of rifles in your cities, they will be quite secure from all but a major attack (unless the AI gets smart and bombards your cities, which I doubt they would do). Cavalry will not have too much luck against fortified rifles (although they can take cities, if you are prepared to take major losses), so I recommend bringing artillery of some kind if you mean to attack cities in which rifles are fortified. Get used to using rifles, because you will see a lot of them.
Paratrooper (6/8/1): This is a specialty unit that rarely has a convincing purpose. Their special ability is to paradrop, which allows them to "drop" a certain distance away from cities with an airport and still move/attack on that turn. It could be potentially useful, except that you can do the same thing with helicopters and not need the airport to be in the city. Paratroopers have only 6 attack, no better than cavalry, only have 1 movement point, and will be facing infantry (if not mech infantry) in cities they try to attack. They are not precisely "useless", but it's rather difficult to find a use for them. You'd be better off using helicopters to drop marines in that kind of situation.
Infantry (6/10/1): Everything that can be said about the rifleman goes double for infantry. They almost completely shut down offensive operations when they arrive, as cavalry are still the best attackers at that point and for a long time afterwards. Infantry do require rubber to be built, but their 10 defense (+25% for being fortified, +100% for being in a metropolis...) will ruin any conventional attacks at the time they are discovered. If you want to attack infantry before the advent of tanks, take along plenty of artillery to bombard them down to size. Without support from bombarding units, infantry will inflict horrifying losses on your attackers. On the other hand, with a bunch of these guys your cities will be virtually impervious to enemy attacks. They usher in a very defensive era in the game, and you had best be aware of that in order to avoid massive losses in units.
Marine (8/6/1): This is another speciality unit that can be very useful in the right circumstances. Their special ability is to attack cities from ships, during which they ignore city defensive bonuses. This might not seem like much, but with that ability marines have 8 attack against the 12.5 defense infantry have when fortified (compared to 22.5 in a metropolis when the city defensive bonus is added). Marines can be very useful in blitz invasions of another continent, such as capturing a city on the coast with marines, then moving the invasion force of tanks into the city and hopping to other cities on their rail network. And marines are the only units that can capture a city on a 1-tile island, such as the ones that exist on the earth map that is packaged with the game. If you can't secure oil for tanks, marines are your best industrial age attack force. They aren't useless at all, just speciality units - keep that in mind.
Tank (16/8/2): In a way, the Industrial Age ends when tanks are discovered along with Motorized Transportation. I say that because tanks allow the player to conduct offensive operations with a realistic chance of success once again. Tanks are strong enough to punch through the defense of infantry without massive artillery support, although they will still take significant losses. One of the few disadvantages of tanks is that they require a lot of resources to build: both oil and rubber. They aren't all that fast with only 2 and not 3 movement points, but tanks are still more than quick enough to get the job done. Get your hands onto these things at all costs if you want to make some noise in the late industrial age.
Artillery (0/0/1) (bombard 12/2/2): I love these things. Artillery is godly in its power, for with enough of them you can capture any city and defend against any attacking force, no matter how large. Although they cannot actually kill units, they are very good at whittling down units and cities so that other units can kill or capture them. Of course artillery aren't very good in small chunks; you need them in large number to accomplish what you want. A stack of about 30-40 of them is usually good enough for offensive purposes; that is, good enough to bombard every defender down into the red and hopefully take the city below size 13. With artillery support, cavalry can take out even mechanized infantry, though hopefully your situation will never be quite that dire. On the other hand, a stack of artillery can utterly decimate an invading army before they ever reach one of your cities. Just remember that artillery cannot defend themselves and must be protected by other units. Build lots of these, and you will not regret it in the slightest.
Fighter (4/4/0) (bombard 2/4/2): I feel pretty strongly that these are nearly useless units in Civ3. Since air combat is random and virtually non-existant in the game, there isn't much purpose to building these things. The only real use for air units is to bombard cities and terrain improvements, which the bomber does much better. If you are having your bombers shot to pieces around a certain AI city I suppose you could build these, but I don't really see the use. Fighters are mostly wasted shields.
Bomber (0/2/0) (bombard 8/6/3): These are excellent support units for your attacking force. Although I have little experience with air units (normally I have won or ended the game by the time they come around), I have seen the devastation that a group of bombers can wreck on an enemy cities. As with other bombard units, you need a lot of these things in order to have a significant effect; I would guess at least 15-20 to really cause serious damage. Their large range is great, although there is the possibility of the city in which they are based culturally flipping back to the opposing civ. I can't give too much advice here on a subject with which I have little familiarity, but I do know that bombers are good units that can provide strong support for columns attcking units.
Helicopter (0/4/0) (range of 4): This is a very strange speciality unit that is rarely used, although in theory could be useful. The helicopter can "drop" up to two non-wheeled units (in this case, likely infantry, marines, or paratroopers) anywhere up to 4 tiles away from the city in which it is based. It's a cool idea - except that tanks have much higher attack value than these units and helicopters require a separate and somewhat useless optional tech to be built. I wouldn't put too much effort into these things, but in the right circumstance they could be handy. They are still more useful than the paratrooper's paradrop ability, since that requires a city with an airport and helicopters will work in any city.
Transport (1/4/5): Almost nothing I can say about these units. They will carry up to 8 ground units, and are the standard for any overseas invasion force in the industrial age. Obviously you have to protect them with other ships if you don't want them to get sent to the bottom of the sea. They are the modern equivalent of the caraval/galleon; a troop-carrying ship. Not too much more can be said.
Carrier (1/8/4): These ships are to air units what a transport is to ground units, only more so. If you are worried about putting your bombers or other air units in a recently captured city for fear of a culture flip, you can always put them on a floating airbase where they will be safe as long as the carrier is. Use carriers the same way you would in the real world: supporting overseas invasions, providing air support in areas that are not secure, when island hopping on archipelago maps, etc. They should be protected by either battleships or destroyers, hopefully a decent amount of both. You don't want to lose your planes, right? Carriers have plenty of uses, and they certainly look impressive (as if that means anything). Be warned that they are very expensive indeed at 180 shields though. If you want to conduct air missions overseas or bombard the living daylights out of your enemy from a position of relative safety, these things are for you.
Ironclad (4/4/4) (bombard 4/1/2): These units are a major upgrade from the frigate (which is why you should never waste time on frigates), though their uses are still rather limited. The best use of ironclads is to protect galleons/transports full of units from attack by enemy ships and to sink the corresponding enemy transports. They cannot be upgraded and will be all but worthless once Combustion brings destroyers onto the scene. They can bombard, but not with enough strength to do much damage to cities or terrain improvements. All in all, ironclads are a big improvement over the frigate, though that is not saying much. Use them to protect your unit-carrying ships, and don't waste resources on them beyond that - there are better ways to bombard your enemies.
Submarine (6/4/3): I'm not really too sure what the purpose is behind these units. They can't bombard, so they are only good at attacking ships - and at only 6 attack, are not much stronger than the 4 defense that a transport gets. Now the fact that subs are invisible to all but a few other units is nice, but not so big a deal as to make them outstanding units (I'm also pretty sure the AI can see them even though they are supposedly hidden). Their only real use is to sink enemy transports, which either destroyers can do much better at almost the same cost. Subs are rather slow too, at only 3 movement. I don't recommend building too many of these; they may have more uses when multiplayer comes out, but against the AI they don't really serve much of a purpose.
Destroyer (12/8/5) (bombard 6/1/2): These units are the workhorses of the sea during most of the industrial and modern age. They are cheap compared to battleships (120 shields to 200), attack and defend almost as well, and can bombard on the side. These are the ships that should be out there protecting your transports and carriers from enemy attacks. When they are not under attack, they can add in their bombardments as well, but real full-scale bombarding is not their strength. I recommend building lots of these in the context of a naval war situation - which is unlikely to happen most of the time, since Civ3 places little emphasis on ships. But when you need a ship to attack another one, or defend against another attack, destroyers are the ones you should probably be using.
Battleship (18/12/5) (bombard 8/2/2): Now these things really are the kings of the sea, and you can tell it just by looking at them. They attack and defend better than just about anything out there, so you will want to have a couple of these around "just in case" when doing anything overseas. The real disadvantage of battleships is their cost; at 200 shields, you could build 2 transports, 6 settlers, or 20 workers! Because they are so expensive, I recommend building mostly destroyers and using battleships as more of specialty units that pack an extra punch when needed. Battleships should probably use their bombard ability as much as possible, and actually attack and defend as little as possible. The things are just too valuable to throw away to the ravages of the random number generator in battle.
Mech Infantry (12/18/2): These are the best defenders in the game. Period. They also are the last leg in the spearman upgrade chain, and the only unit in that group to have two movement. As the default defender of the modern age, they are also the unit that you will conscript once you discover computers. Unlike other conscripted unit from early ages though, they have both enough attack and movement to pose a serious threat on offense. Be very, very wary of these units, as they can be all but impossible to dislodge when fortified in metropolises (defensive value of 40.5, plus whatever the terrain bonus is) and can pose a significant threat on offense as well. Try to get these things as soon as you can in your cities if a game is still going on in the modern age. I'd also recommend sending some along with your attacker on offense to provide defense and garrison duty; in most situations they can keep up with attacking units. This is one of the best units in the game.
Modern Armor (24/16/3): These are the best attackers in the game. Period. The three move lets them move at an amazingly fast speed and they defend almost as well as mechs. The main problem is accumulating three separate resources (oil, aluminum, rubber) to build the dang things. Once you can get them though, there's nothing anyone can do to stop you. Build a ton of these and you should be able to take out just about anything (as long as you don't do anything stupid, like overextending yourself or taking on too mant foes at once). If your opponents have modern armor, I'd suggest trying to deprive them of one of these resources so that they can't build them. The modern armor is highly superior to the tank in every way, and you should rightly make use of and have respect for these things.
Radar Artillery (0/0/1) (bombard 16/2/2): Now this unit is all but useless. They come so late on the tech tree that I have never used them in battle, and they offer only a minimal increase in strength over the artillery unit. Build them if you are still playing the game at that time, but you rarely will get the opportunity to use these. Normally the game will have ended by spaceship or some other method first.
Cruise Missile (0/0/1) (bombard 16/2/3): A very unusual unit (if I can call it a unit) that can be useful late in the game. The cruise missle is like a one-shot bombardment; it fires once and then is dead. So what's the use? Unlike other bombardment units, cruise missles can actually kill their targets. And at only 60 shields each, they can be produced in most core cities in one turn. Since the AI loves to send ships to bombard your terrain improvements (for no reason at all), I like to bombard their ships into the red with conventional artillery and then kill them with a cruise missle. Kill enough ships this way, and they will get the idea. You can also send cruise missiles along with attacking units, but their slow movement and limited bombardment range (?) prevents them from really being much of a use on offense. Their best use is probably on defense, used to kill units injured with non-lethal artillery.
Tactical Nuke (0/0/1): I have never used nuclear weapons so I don't know what to tell you with these. I suppose they are good for killing units and wrecking cities, but... using them is sure to gain the enmity of the entire world and cause massive pollution. If destroying the world is your cup of tea then go ahead and use them, but be warned that you need to build the Manhattan Project wonder (or have another civ build it) first and deliver the nuke close enough to the target for it to be of any use. My only use for nukes is to use them as prebuilds for spaceship parts (which they are very good at, costing 300 shields).
ICBM (0/0/0): These units are like nukes, only they cannot move and deliver automatically to their target. Keep in mind that they have to stay in the city where they are built, since they can only move by firing. ICBMs are best used as spaceship part prebuilds, unless you are trying to destroy the world (which I won't recommend). Use them on other civs at your own risk. They are ungodly expensive at 600 shields (same as Sun Tzu's).
Jet Fighter (8/4/0) (bombard 2/6/1): These things are just an upgraded version of the fighter - which means they have few uses. If you want an air unit to bombard, build a bomber instead (they're much better at it). I suppose that if your bombers are all getting shot down you could build these things to help protect them, but I don't see that situation coming up very often. But hey, maybe you are more creative than I am and can find a use for these things. I don't see much of one though.
Stealth Fighter (0/0/0) (bombard 4/6/2): These things are misnamed, for they are actually bombing units, not fighters. The stealth attribute makes them almost impossible to intercept (I believe the chance is something around 5%), so you won't have to worry to much about them getting shot down. With that said, their bombard value is still only 4, which makes them nowhere near as good as the conventional bomber. My suggestion is to build a normal bomber or a stealth bomber and not waste time with these things. Since stealth bomers are incredibly expensive though, if you want a cheaper version of one, go with the stealth fighter.
Stealth Bomber (0/0/0) (bombard 8/8/3): Awesome. That's all I have to say about these things. They cost a ton of shields (240!), but they are almost impossible to intercept and have a huge range to hit from (8 tiles). I think you can upgrade normal bombers to these (not exactly sure on that), although only at extreme cost. It is worth it though, because these things are flat-out the best bombard units in the game. Unfortunately, they come so late on the tech tree that virtually every game you play will be over by the time they appear. Still, if you get the chance to build these you should certainly do so. You won't be disappointed.
Nuclear Sub (6/4/3): The only difference between these things and a conventional sub is the ability to carry one tactical nuke inside. They can't attack or defend worth a darn, so don't waste time building these things unless you mean to hit someone with a tactical nuke. Because of that, I have virtually no experience with these things. They don't really do much that conventional naval units can't do, so don't waste your shields on these things unless you are going to nuclear warfare.
AEGIS Cruiser (12/10/5) (bombard 4/2/2): Another virtually worthless unit. They also come so late in the game, you will almost never see them in use in any kind of meaningful fashion. They are a more expensive than destroyers and provide very slim benefits over them, the only real one being the ability to spot subs. Maybe they'll be useful in multiplayer games, but they have few uses at the moment.
That wraps up this exhausting coverage of the units. I feel like I gave some of them less space then they deserve, but most knowledge of what units are good and bad comes from playing the game. There's only so much that I can say here in abstract fashion; mostly my impressions on the usefulness of each one. You will have to draw your own conclusions when playing the game yourself. But don't expect to find results too unlike what I have posted here; I have played this game a lot, and my comments are based in long experience. If I've managed to give you a general feel for what units are good in each era, then I've done what I set out to do here.
The next chapter will cover different technological advances across the 4 eras.