A moat? was a common addition to medieval fortifications, and the principal purpose (just as in antiquity) to make the walls harder to assail and increasing their effective height. In many instances, naturual waterpaths were used as moats, and often extended through ditches to surround as much of the fortification as possible. To position a castle on a small island was very favourable from a defensive point of view, although it made deliveries of supplies and building materials more cumbersome and expensive.
To facilitate transportation but still maintaining the advantage of the construction, a drawbridge? was often constucted as a part of the bridge spanning the moat.
At this time, stair were generally winding, and constructed as to give a defensive edge to a defender, often positioned higher that an assailant who presumably entered on the ground floor. On the general principle that most people are right handed, the stair was constructed right turning, forcing the assailant to fight with his sword hand close to the central pillar of the stair, thus limiting his ability to manouver and attack.
As the power of cannons grew during the 16th and 17th century, medieval walls became obsolete as they were too thin to offer any realistic protection against prolonged bombardment. As a consequence of this, many walls from medieval times were torn down and the stone (still valuable as construction material) reused in more modern bulwark?s and bastion?s. The resulting space is often seen in old city centers of Europe even to this day, as broader streets often outline where the old wall once stood (evident is for example Prague and Florence).
See also: Medieval warfare, Medieval siege weaponry, [Medieval naval warfare]?
Siegecraft: