The "hon-maru" is shaped like an abbreviated form of a cross because of the complicated exterior lines of stone walls. The "de-sumis" (salient corners) on the north side of the "hon-maru" are rectangular flanking projections which increase the castle's defensibility; the "iri-sumis" (receded corners) on the south side of the "hon-maru" also increase the castle's defensibility because they permit cross fire at the enemy at the southern corners of the "hon-maru". The top of the foundation of the "dai-tenshu" (large donjon) is 36.3 metres (119 ft) above sea level, and at the centre of the "hon-maru", 23 metres (76 ft) above sea level. The "hon-maru" of Fukuoka Castle was divided into a northern part and a southern part by the stone foundations for the "sho-tenshu" (small donjon), the "chū-tenshu" (medium donjon), the "dai-tenshu" (large donjon), and the "kanritsu-shiki tenshu-kuruwa" (a "kuruwa" formed of rectangular buildings as the final strategic position in time of siege). The southern part is apparently the "tenshu-kuruwa" (a "kuruwa" especially designed for the defence of the "tenshu"), playing the role of the "tsume-no-maru" (a "kuruwa" for the final fighting of a siege), and it is the most fortified place in the castle. On the northwest corner of the foundation of the "tenshu", the "tenshu kuruwa" has in addition the "kanritsu-shiki tenshu-kuruwa". This particular feature indicates that the castle's purpose was defensive.