Stemming from the original Norman castle, the Crusader concept of castles primarily depended on fortifications of a great citadel built on a natural or man-made eminence. The Ismailis, whenever possible, fortified the crown of a great mountain, dividing the fortifications up into self-contained sections, culminating in a great citadel.
The decision to fortify or build a new fortress on an existing site was based on four main principles:
- The area chosen for fortification must be in a naturally strong defensive position with a terrain sufficiently remote and difficult to approach in order to discourage attack hostile parties.
- The complex of fortresses within the chosen area must have the ability to support each other in the event of an attack and enable an efficient system of communication to be established, whether by beacon or other means.
- The chosen area must contain enough material, especially wood and stone, to allow the construction and reconstruction in the case of an existing fortress, to be carried out expeditiously and with a minimal labour force.
- The terrain should have fertile ground and water nearby to provide adequate water and food supplies.