The term "building" refers to what you can build or extend in the village centre. For travian advice go here
The wall can be built only in the outer area that includes the moat. The rally point can be created only in the space to the right of the central circle. Any other building may be built on any vacant circle/oval, provided that its prerequisites are met.
It is possible to have more than one of a particular building, as soon as the first one is complete. The first such duplication is nearly always the cranny; after the first is complete, it is good value to build a level 1 cranny in several other vacant spaces before taking one to level 2. A second warehouse is needed before you can extend a palace to level 20.
Speed of erection or extension of any building is related to the level of the main building, and is cut to 20% of the standard if the main building is destroyed. So rebuild your main building as soon as possible (on any vacant circle) if it is destroyed; in Travian2 that requires you to have a positive crop production rate, which may take a very long time if your croplands have been destroyed too. For travian advice go here