A spur castle is a type of medieval fortification that uses its location as a defensive feature. The name refers to the location on a spur projecting from a hill. Ideally, a spur castle would be defended on three sides by steep hillsides, with the only vulnerable side the one where the spur joins the next hill.

Depending on the local topography, a spur castle may depend mostly on its inaccessible location, or combine it with defensive features such as walls and towers. A typical feature is a neck ditch cutting off the spur from the rest of the hill. When the spur is long and narrow, the term ridge castle is sometimes used as well. A long spur castle is sometimes, but not always, subdivided into a lower ward and a more strongly defended upper ward (or even a succession of more wards).