A keep is a square tower, the strongest and most secure part of the medieval castle. Often, because it is the nucleus of the castle, it will be the most heavily guarded. Because of this, many keeps have crenelations surmounting their walls for defensive combat. Sometimes, as is often portrayed in Hollywood, a wide trench filled with water would serve as an added fortifier for the keep.
Also, as designs were modified, keeps took on different shapes. The variations on the keep theme include the square, rectangular, cylindrical, and polygonal designs.
In France, a keep would be called a donjon, of which the English word "dungeon" is a cognate.