Wondering what to build in your second, third and fouth cities? Here are some suggestions on what is necessary and what isn't.
The quick answer is the simple recipie at the bottom. To learn why, read all of this page.
Which buildings and how many Cottages or Barracks depends on what your city will be used for. There are many strategies on how to compose and combine your cities. One suggestion is to have one city for making troops and one city for making gold. In both cases you'd want to have as much Cottages, or as much Barracks, as can be afforded. Having more than one city also carries the risk that someone conquers your city and takes it from you.
You definately need to build the following in all of your cities:
Academy: Every city shares research, up to the level of the city's Academy. For instance level 2 is needed for resource production bonuses.
Beacon Tower: Every city, you'll want to know if you have enemies coming.
Cottages, several. You will obviously want several of these in every city to allow for population growth.
Rally Spot is used to send troops to and from the city. The first thing you should build in a new city.
Barracks: Again, you will need several of these if you intend on building an army.
These are useful but not always needed for your secondary cities:
Forge and Workshop: Forge is needed to build a Workshop so you can build your Wall. After you're done with your walls, you could consider replacing them with Cottages or Barracks. Keep a high level Forge in the city where you are researching Military Science.
Inn and Feasting Hall: You will want one if these in every city in order to acquire and appoint heroes for each city. Since it is possible to Reinforce a hero from one city to another, you could have a high level Inn in only one city, and all your other cities can save that space for an extra Cottage or Barrack. In a new city, it is possible to survive for a short time with no Inn or Feasting Hall at all: Reinforce a hero into the new city and go to conquer some valleys. When that is done, Reinforce that hero back to a town with a Feasting hall if you want to upgrade the hero. But since you should always have a Mayor in each city, that means you need to build a Feasting Hall to assign a hero as Mayor, and after that you can (probably?) not remove the Feasting hall.
Stable: Needed to build Relief Station. Also it is claimed to help with development of mounted troops.
: Is used to save time on transports from your city. If your cities are close together you might only need one in your main city, if at all.
: An embassy in your second city is only useful if you have helpful alliance neighbours and activate Allow garrison. Then upgrade to have at least one level per helpful neighbor.
Warehouse: Protects your resources from Plundering. If you build a strong army early on and keep city gates open that can be enough protection. Otherwise you will soon need at least one. You could consider waiting with upgrades or secondary warehouses until you start getting attacked.
: Is often used to sell Lumber and buy Iron and Food. You pay a gold fee for all transactions. When you get attacked, you can sell your unprotected resources, and then buy them back just before the attackers arrive. If you time it right your new resources will arrive safely soon after the attack is over. If you have a marketplace in a nearby city, then you can transport gold to the marketplace city and then transport resources back out but it means some more work for you. If your cities are more than 30 minutes apart then build marketplaces to avoid having to levy resources.
Simple recipe -- If you want to build your cities far apart so they can function independently:
Main City: 1 rally spot, 7 or 8 barracks, 12 cottages, 1 Beacon Tower, 1 Embassy, 1 Academy, 1 Inn & Feasting, 1 Forge and Workshop, 1 Market, 2 Warehouses, 1 Stable & Relief Station.
Second City & So On: 1 rally spot, 7 or 8 barracks, 14 or 15 cottages, 1 Beacon Tower, 1 Embassy, 1 Academy, 1 Inn & Feasting, 1 Forge and Workshop, 1 or 2 Warehouses, 1 Stable & Relief Station.