A castle is a keep usually surrounded by a 15-foot stone wall with four towers. Huge castles have a particularly large keep with numerous associated buildings (stables, forge, and the like) and an elaborate 20-foot high wall creating a bailey and courtyard areas. The wall has six towers. Castle walls are usually ten feet thick. Castles are the centers of some of the largest humancities.
In the Warcraft RTS games, castles are upgrades to keeps.
The mighty castles of Lordaeron are the center of large military cities. As with the lesser keeps, peasants may bring gold and lumber here for judicious dispersal in order to sustain the war effort. These bastions against invasion are guarded by fortified battlements, making them virtually indestructible. The castle represents the strength of the peoples of the Alliance and their unyielding resolution to defend the whole of humanity against the Orcish Hordes.
Castles represent the pinnacle of advanced human civilization. With this advancement comes the ability to support stables for housing the war-steeds of the heroic knights. This progression also allows the building of gryphon aviaries.