All defensive structures have to have an entrance of some sort. From the beginning, it was recognized that any entrance was a very possible weak point in the defences. Considerable ingenuity was thus employed in their design and construction. The first step might be to provide a tower either side of the gate so that any attackers could be fired upon from a position of greater height. Making the route to the gateway as convoluted as possible also helped. Making any bridges which spanned ditches removable was an added refinement. This concern for making the entrance as difficult a point to breach as possible continues to this very day.

There was, however, a period during the evolution of the medieval castle in which this concept was taken to an extreme. For centuries beforehand, the keep had been regarded as the final strong-point of the defensive plan, and the gateway, or gateways, were part and parcel of the outer defences, the curtain walls and towers. Now there was a concentration on the gateway itself. In effect, the roles of keep and gateway were combined in one building, the "keep-gatehouse". Doune, in Scotland, is my personal favourite when it comes to giving examples. In these cases, the inner curtain wall now protects mainly the ancillary, domestic buildings of the castle. In Scotland, perhaps the development of the towerhouse is the next logical step, with even these incorporated into the one structure.