The Belém Tower was built on the north bank of the Tagus in 1520 AD as part of the estuary defense system, and is one of the jewels of architecture during the reign of Manuel I. Walking around the tower as a whole, two different sizes of military architectural models can be distinguished: one is a medieval tower and the other is a modern fortress that contains two levels of cannons, allowing for long-distance firing as well as recoil shots over water. As the protector of Portugal, this tower witnessed great importance in 1983, when UNESCO classified it as one of the cultural heritage sites of humanity.