Speaking of fortification as science is only possible when it associates to build a defensive technique. At the end of the 20th century. XV, the medieval fortification, due to the emergence of pirobalística artillery, began to become ineffective. The evolution has become unavoidable, being gradually replaced by a castle Fortress. This transition was by distinct ways in which, for example, binding a medieval tower a bastion. In Portugal, this transition was led by the Italian school, as in the rest of Europe, and will materialize in the building of the Tower of Belém, in the reign of King Manuel I.
Source: Official Website