Safety, not style or comfort, was the most desired quality. The castle had to be able to withstand an attack or a siege.

In the medieval era, the most desired quality of a "good" castle was defensibility, specifically the ability to withstand a prolonged siege. While modern eyes see castles as romantic palaces, their primary function was as a military fortress. This was achieved through a combination of "passive" and "active" defense features. The most critical aspect was the site selection; a castle built on a high rocky outcrop (a "motte") or near a steep cliff was significantly harder to attack than one on flat ground. A "good" castle also required an independent and secure water source, such as a deep internal well, because a garrison could be forced to surrender in days without water, regardless of how much food was stored. Beyond physical thick walls and moats, the "concentric" design—where an inner wall is higher than an outer wall—allowed defenders to fire over the heads of their own men on the outer perimeter, creating a deadly "killing zone." Ultimately, a castle was only as good as its ability to make the cost of an assault too high for any enemy to consider.