It had several security zones. Sperrkreis 1 (Security Zone 1) was located at the heart of the Wolf's Lair. Ringed by steel fencing and guarded by RSD and FBK men, it contained Hitler's bunker and ten other camouflaged bunkers built from 2 metres (6 ft 7 in) thick steel-reinforced concrete.[73][74] Sperrkreis 2 (Security Zone 2) surrounded the inner zone.