Before its alteration and completing in the 1850's the North centre and Detached bastion had an earlier phase of work with a different layout. It early plan resembled a triangle with a truncated tip, with longer east and west flanks tapering to a shorter northern face. It was an unrevetted earthwork which had incorporated an earlier linear fieldwork, dating probably from the American war of the 1770-1780s. the huge ditch and rampart of the bastion were constructed across this older work, producing two tiers of defence. (location accurate to the nearest 50m based on available information)