During the Antiquity and later on in the Middle Ages the powerful rulers were constantly fighting each other. Therefore it was necessary to protect their possessions. Usually they surrounded themselves by deep defensive moats full of water. Behind the moats they had thick and tall walls, made of large stone blocks. We call them defensive walls. You can reach the fortress only through the hanging bridge over the moat. Upon attack this bridge was raised and the fortress immediately turned into a place, difficult to access.
There is a large variety of fortresses, yet there are some features, which can help you recognize them.