Until the late 12th century castles generally had few towers; a gateway with few defensive features such as arrowslits or a portcullis; a great keep or donjon, usually square and without arrowslits; and the shape would have been dictated by the lay of the land (the result was often irregular or curvilinear structures).

The "average" castle evolved significantly over time, but the most common image is based on the stone fortifications of the 12th and 13th centuries. These structures were primarily military machines designed for defense rather than luxury. They typically featured a central Keep (a massive, fortified tower), surrounded by a Bailey (an enclosed courtyard) and protected by high Curtain Walls with towers at the corners. Defensive features included moats, drawbridges, and arrow slits. Inside, the "Great Hall" served as the center of social and administrative life, while the living quarters were often damp, dark, and drafty. In 2026, many of these ruins are popular tourist sites that showcase the transition from early "motte-and-bailey" wooden mounds to the complex, concentric stone fortresses. It is a peer-to-peer essential to remember that for most of history, a castle was a crowded, busy village-within-walls, housing soldiers, servants, and livestock alongside the nobility.