Solid BASTIONS, are those that have the void space within them filled up entirely, and raised of an equal height with the rampart.
Void and Hollow BASTIONS, are those that are only surrounded with a rampart and parapet, having the space within void and empty, where the ground is so low, that, if the rampart be taken, no retrenchment can be made in the centre, but what will lie under the fire of the besieged.