Although castle architecture developed over the centuries in line
with weapons technology, the principles remained much the same.
A good castle provided a secure base that could be easily defended.
It needed defenses against frontal attack (thick, high walls and
secure entry gates) and from undermining (rock foundations or a
It furnished means of repelling attackers while minimising exposure
of the defenders (arrow loops, crenellation, machicolations, murder
holes). It also provided means of escape and of making sorties against
attackers (postern gates and secret tunnels).
Further, it needed facilities to withstand a siege - a fresh water
supply or large cistern and vast supplies of food.
A good castle had no dead-spaces - ie external areas that defenders
could not fire on, but did provide multiple locations from which
vulnerable points could be defended.
The best castles provided rings of defence so that defenders could
hold out from a citadel even if the outer defences failed. Not least,
a good castle was strategically placed, for example to monitor and
control access to an important route through a valley.
Castles also needed facilities for a garison and living quarters.