Links to all Building Pages
The Keep is the Main building in your City. It is REQUIRED to be upgraded before any building can go to a higher level. NONE can go higher than the Keep. You will see references to Keep level this and that as a reference point because of this. You can Levy daily for gold among other things here.
The Walls The Walls need upgrading before the keep can be upgraded EVERY time. They also need to be upgraded before the Archer Towers can be upgraded. This is where you Assign a General to protect your city and Patrol the wall to get some goodies daily.
The Academy The Academy can be built at Keep level 2. It is Important to keep this at the highest level possible at all times because the research is often a pre-requisite to a Keep upgrade and research is a very large part of the game play success.
The Rally Spot The Rally Spot tells you the basics about your troops and traps and your March size. You can upgrade from the beginning along with your Keep.
The Market The Market you can start building at Keep level 6. It is ALWAYS required to be upgraded before the Embassy. This is where you tax your resources daily and find the Black Market.
The Embassy You can build the Embassy at Keep 9. I would suggest this as another building to keep as high as possible as it is where you accept reinforcements from Alliance members in case of an attack on you or you sub cities. As well as the tax for sending resources drops the higher level of Embassy that you have. The Market ALWAYS needs to be upgraded first.