This is a guide written by Asjo for the .EN forums.
Once you have a few cities, you have to think of how you want to use them. You could have every city in a balanced state of defense and offense, but there are certainly uses to city specialization.
There are limits to this game that urge you to be effective. You cannot move troops between cities, only support, and there is a maximum population to each city, so your army can only be a certain city. By specializing cities, you can make sure to have the optimum amount of attackers and defenders where you need it and you can get to pick the techs that fit the strategy of each city (which is useful, since it means you don't need to build Library to get all the techs you want)
When you attack, it's often beneficial to have a lot of troops of one attack type. So, it would be nice the city that is nearest to your targets could spend all its population on slingers. If you build thermal baths as one of your special buildings, your maximum population once you reach farm level 40 will be 3500. At least 500 population will be tied up in buildings. Ideally, that would leave enough population for you to build 3000 slingers to attack with. However, since you're not attacking on your own island you will need transport ships. Normal transport ships can transport more troops for their population cost, however, since they're very slow, they will normally not be a valid option if you plan to use your city for raids, not just conquests (with conquest, the speed doesn't matter, since colony ship is slower). You will have researched Bunks, which means that each fast transport can carry 16 units. If you have 3000 population available, you would be able to make 2280 slingers and build 142 fast transports to transport them. However, the problem is that if you don't have any attacking ships escorting the transports, they will all die when faces with any ships from the defender. A part of the offensive strategy is to have a city where you build only light ships to use for attack. The light ships can be used to screen for defensive ships so that your conquest army won't face any. When you attack, you can time your attacks from the different cities so that the attacks with the light ships hit just a few seconds before your main attack. Having all your light ships in one city will mean that you only have to time attacks with light ships from that city, not all your cities. Still, you should always send some light ships along with your main army so that are never completely dead if the defender intercepts with defensive ships. With a maxed army worth 3000 population, you could have 40 light ships, 1950 slingers, 121 fast transports.
40 light ships is a good number if you're just looking to use your offensive city to raid for battle points as well. It's unlikely that anyone will keep more than 40 biremes in one city. So, to defeat you, they have to send biremes from other cities as well. If you're fairly close to the target, it's unlikely this will happen, especially if you have sacrificed the 400 extra food from Thermal baths to build Lighthose instead, giving your fleet a 15% speed boost. If they do manage to get, say, 100 biremes to defend, those will only manage to kill off 40 of your transport ships since they will have to fight your light ships. If your target and his nearby friends have too many biremes, you might want to use your city with only light ships to kill some of those a few days ahead of the attack.
A city with only light ships can be a bit risky, since, if attacked, they will be very easy defeat. If you choose this strategy, you should be certain that every time your are not online, you send your light ships to attack an inactive player far away so that the light ships will return from the attack at the time when you come back. If you are offline for six hours, you should attack a target three hours away, making the trip back and forth six hours in total. If you can do this, light ships are also a great defense against conquest. If anyone conquers your city while the light ships are away, they will fight him when they come back. They will likely have great enough numbers to defeat the biremes that he has sent to support the city (3000 population = 300 light ships). Even though they don't destroy the land troops, they destroy the colony ship, which means that the conquest attempt is over.
In the strategy described above, there are no troops to defend the cities! Won't you get conquered or your offensive troops easily destroyed? Well, yes, it does require a bit more attention since you have many offensive troops and don't want those caught in defense. However, if you have a defensive city, it will send troops to defend your offensive city, thereby allowing you to use all your supply on offensive troops. If you are very active, you probably don't even need that, since you or your allies will be able to send biremes, defensive troops or light ships to stop a conquest at any time. However, if you don't want to risk anything or simply don't have any friendly cities nearby, you send enough defensive troops to your offensive city to be able to effectively defend against or at least deter a conquest attempt.
As a defensive city, you should build Thermal baths and Tower as your special buildings. With Tower you defend more effectively (adds 10% strength to both land and sea defense and cannot be destroyed, unlike city walls) and with Thermal baths you have an additional 400 extra population available. There are two types of defensive cities. One that produces defensive troops. And one that produces biremes. While biremes are limited to defense, it does have three distinct advantages over the light ship:
1) With biremes you can defend your city from conquest passively. If you have more biremes in your city than your opponent can kill, it will be impossible for him to conquest your city as you will kill his colony ship. You don't have to actively defend - as long as the biremes are there, you city is safe.
2) While light ships can help keep your own fleet safe, biremes can tear up an offensive fleet. If your biremes sink a lot of transport ships, killing all the troops in them as well, they suddenly become extremely cost effective.
3) Once you grow very big, you will suffer from bad morale when you attack players who only have a fourth of your points. Therefore, fire ships are practically limited to attacking bigger player. However, if that smaller player decides to attack you, your biremes can defend without any penalties.
An added advantage to having pure defensive cities is that they are much easier to keep safe. If you only have defensive troops as well as a level 25 city wall and tower, it can be extremely tough to break the city. If you reserve half of your defensive army for supporting other cities, you can keep the ofter half of your defensive army in medusas, for instance. Medusas, which you can build if you worship Hera, aren't particularly cost-effective defenders (even if you disregard the fact that they cost favor), but, after cerberi, they are the most population-effective defenders in the game, and a pretty good attackers as well (giving you the option if it becomes necessary). They have balanced defense attributes (only slightly weak against slingers). By building many medusas (probably aided by some swordsmen to deal better with slingers), you have a very population-effective defense since an equivalent defensive force with swordsmen, archers and hoplites would have cost more population. This frees up some population to send to defend other cities. The only disadvantage to this is that you cannot send the medusas to support any city which does not worship Hera. If a crisis situation, of course, you could ask the person you need to support to change the God in said city. If you send your medusas to support a city, it will have to change its God before their arrive, otherwise your supporting fleet will turn back.
One thing you should keep in mind when having multiple cities is not to stack all your cities with the same God. You will want to have all the different God be to able to cast different spell on your attacks to protect them (if you have cost a spell on your attack, it cannot be affected by spells from the defender). But also keep in mind that when you have more than one city worshiping the same God, the cities do not produce favor as effectively as they otherwise would have, so you're wasting favor by having less diversity than you otherwise could have.
Distribution of cities
So, when do you do specialize cities and how many do you make of each? Obviously, you first city will have to be a mixed city that can both attack and defend. However, you can start specializing cities already when you get your second city. If you're in an area of a lot of new players, you might have enough units in your first city to break their defenses, but a pure bireme city might prove invaluable to holding your siege against opponent who only have a few cities as well. It is all about adopting to the needs of your situation, but it is almost always advantageous to specialize right away. If you second city is a defensive city, your first one can be turned into an offensive city. If you are attacked often, the defensive city can split its troops between your two cities, while the city that is purely offensive can make stronger attacks by attacking in numbers. One advantage, however, by having cities with a bit of everything is rebuild time. If your defense is spread over several cities, you can more quickly rebuild if you lose it all. Therefore, it is often advisable for less active players to wait with specializing their cities until their have at least seven cities. At that point, you start having enough defense for more than one attack. With seven cities, you could have two bireme cities, two defensive ground cities, two light ships cities and one ground attack city, which would often be enough to attack and defense effecitvely against opponents of similar strength.
This tactic allows you to make more large-scale attacks closer to your enemy. It makes conquest easier and gives you the advantage of having big sea armies in a single city. The pitfall of this strategy is that you have to be an active player to minimize the risks involved and will not be able to handle your defensive army as effectively if it's on support (you won't be able to dodge an attack to defend against another, for instance). In the end, clearly the best way to use this strategy is to have enough active alliance members nearby that you can always get any help you need, offensive or defensive.
Another possible problem with this tactic is the distance between cities. Keep in mind that if you spread out your cities, your defensive cities should be those the farthest away from the action, while the offensive cities should be as close to the action as possible.