There are a few medieval fortresses which are pointed out as being prime examples of "concentric castles". This is a bit simplistic. The concept of "concentricity" is the provision of a series of obstacles through which an attacking force, "the enemy", must penetrate, no matter from which direction they launch their assault. This concept was understood and applied by the Romans, and before them, in Britain at least, by the builders of Iron Age and pre-historic hill-forts. Maiden Castle, an Iron Age hill-fort par excellence, is defended by a concentric series of massive ditches and ramparts. The standard-issue Roman fort has a stone curtain, a berm and the outer vallum (ditch for non-latin speakers). The medieval castle-builders did not invent "concentric castles", they just made them bigger and better.

At its most simple, a tower, or "keep", surrounded by a curtain wall is an example of the concept of "concentricity". In this case, the enemy must first breach or scale the curtain wall before they can assault the tower. As the height of the tower is greater than that of the curtain wall, any enemy forces on or within the curtain will be subject to fire from the defenders within the tower, and particularly from those on the tower battlements. To increase the efficiency of this system is simple. Firstly, you add more curtain walls, making sure that the further away from the keep they are, the lower they are. Thus each wall can be fired upon from "the next one in". Secondly, you position the gates in the walls, (always possible weak points), in such a way as to force the enemy to make as long a journey as possible between each one, receiving fire on the way, of course. You can vary the type of obstacle, it doesn't have to be a wall, a moat will also suffice.