Alberobello is well known for its extraordinary roof structures called Trulli, these were an early instrument of tax avoidance. The trullo is a cylindrical shaped roof structure made of dry rock, it was introduced by peasants in 1 and 2 BC to try to avoid the heavy taxes imposed on them by the administrations. The peasants would, however, be allowed to own temporary structures without a levy of tax being imposed on them. The trulli are constructed with a keystone over the front door, the removal of which causes the whole structure to collapse. This allowed them to quickly pull apart their houses when they could see the tax man approaching.