Numerous fortifications from the Saxon era and earlier are still evident across the UK, all now just earthworks, since the original timber structures within have long since disappeared. Stone castles date from after the 1066 Norman invasion, when over one thousand were built, to consolidate the newly acquired English territory and provide the basis from which to extend the kingdom into Wales. Most were still begun as wooden defences but soon strengthened, and often an earlier Saxon fort would be incorporated into the new structure. Most early castles had a simple design, with a circular ditch and embankment, the motte, enclosing a raised central area, the bailey, in which the wooden buildings were constructed. Typically the first masonry component to be added would be a central keep or tower, followed by stone walls around the bailey, commonly with towers or turrets at intervals. Subsequent developments involved additional stone buildings and often an outer defensive wall beyond the original, creating a larger fortified area, the outer bailey.


The main period of castle building lasted about 300 years. Castles were situated in most large towns and beside strategically important locations like river crossings and intersections of travel routes through valleys. Defensive needs declined from the 14th century onwards, when many castles were gradually abandoned, eventually left to become ruinous, or dismantled in order to reuse the stones in other projects, while others were modified for solely residential use. Another major phase of destruction occurred during and after the Civil War of 1642-1651; many were greatly damaged during the fighting while others were slighted (deliberately destroyed) by order of Parliament, to prevent any future use in conflicts.

Today, a very small number of medieval castles are still intact, essentially unmodified and inhabited as private residences; Berkeley Castle is one notable example. A few more are also mostly complete, with some furnished rooms, and preserved as museums, such as those at Caerphilly, Caldicot and Portchester. The remainder exist in stages of ruin, from the barest remains for example Dinas and Morlais, to extensive, spectacular structures of which perhaps the best, in the southwest, are Chepstow Castle, Coity Castle, Corfe Castle, Grosmont Castle, Montgomery Castle and White Castle. Although ruined castles are still numerous, there were many more, generally smaller, for which no trace remains.


As well as the true castles, built for military use, there were quite a few buildings named castles that were instead designed as fortified residences - mansions, or manors - with defensive elements more for show than for practical purposes; they are generally a little younger in age than the actual castles, typically from the 13th to 15th centuries, though they are often equal in terms of photogenic appearance and impressive architecture. Notable examples include Old Beaupre Castle, Old Wardour Castle, Raglan Castle and Sherborne Old Castle

Many medieval castles with significant remains are preserved as visitor attractions, some privately owned but most in the care of the two national bodies, English Heritage and Cadw (Wales). The National Trust owns a selection of the fortified residences, generally complete and often still occupied. Just a few medieval castles are not obviously managed, sometimes remote and overgrown, for example East Orchard Castle in Vale of Glamorgan.