The Keep is your corner stone of your City, without it, you cannot build anything else. Should it be destroyed in a seige, you may lose your entire city.
The Keep is the heart and soul of the village. This building must be built to claim an area as yours, and, without it, no other buildings can be raised. It is vital to protect the keep during sieges. You may lose your entire village if your keep is destroyed in battle. The keep is upgradable, providing many more benefits as you put more resources into it.