Your personal defense protects your buildings, too. Defense buildings are ok to increase your defense stats, but they're not as effective as defense gained from items. An item's defense is exactly what is listed (plus any modifiers) whereas a building's stated defense doesn't match its effective defense; this is especially true of upgraded defense buildings. There are other posts and tables that will allow you to see a building's effective defense at any level. They still provide protection, but just not as much as the "stats" tab tells you. If you have the space and the time, build defense buildings, but they're a poorer investment than items.