It is a good idea to have as many barracks as possible to generate troops faster. So some buildings can be replaced with barracks.
Town Hall
Every city starts with a Town Hall already built. Inside the Town Hall, you can set production rates, view any valleys the city controls, view and abandon any cities you control, and set the tax rate in your city. You may also rename your city, levy the city for resources, and provide comforting to the city.
Cottages
Cottages are built to increase the population of your city. You should keep at least 6 cottages in all your cities.
Embassy
The Embassy allows you to create alliances and allows players in your alliance to reinforce your city.
Academy
The Academy is where technology research is done. You must keep academy of same level in each city. If you keep higher level academy in one city and low level academy in another then the city with lower level academy will not get the benefits of high level researches.
Rally Spot
A Rally Spot is needed to send troops outside of your city. This is also where your troops are healed after battle. Without a rally spot you can not send troops to another city or attack.
Barracks
Barracks are necessary for the training of troops. You should build as many barracks as you can. You only need to build a lvl 10 in one city to build Catapults.
Beacon Tower
The Beacon Tower provides information on attacking troops. Without a beacon tower , you won't have information about the attacking troops.
Workshop
A Workshop is required to build walls and for upgrading your hero's gear.
Relief Station
Relief Station allows troops to move faster between your cities and your ally's cities. without a relief station sending troops to another city will take a very long time.
THE FOLLOWING BUILDINGS CAN BE DELETED
Warehouse
Warehouses provide protection for your resources when your city is attacked. They not really necessary if you have more than one city . When under attack you can send gold and food very quickly to another city using scouts.
Inn
An Inn is where you can recruit heroes. The level of the Inn is equal to the number of heroes that are available for recruitment. You can recruit heroes from the inn and reinforce them to other cities. So , keeping an Inn in every city is not necessary.
Feasting Hall
The Feasting Hall holds the heroes that you have recruited. Each level of the Feasting Hall allows you to hold one hero. If your city is only for producing troops and you never attack anyone or farm NPC's then there is no need for keeping a Feasting Hall in that city.
Marketplace
The Marketplace is used for the buying and selling resources. As you build more cities, markets are only necessary if you are going to trade goods. You can keep a market in one of your city and delete them from the other cities. You buy buy resource in that city and send them other cities.
Forge
A Forge is required for researching higher level technologies. Forge is required for researching Military Science. When your Military Science reaches at 10 you can delete forge.
Stable
A Stable is required for researching some technologies. You can build one stable in one city for completing the Horse back riding and upgrading relief station. Research Horse Back Riding only in the city where you have the stable. Once you have level 10 Horse Back Riding you can delete the stable.