 The forward compound was split into lower ( C ) and upper ( B ) parts by a wall linking towers 4 and 7 . The entrance ( II ) is in the lower part , guarded by towers 8 and 9 . Tower 8 has , in turn , been fortified with a cannon port . Opposing the entrance was a second portcullis that led to the rear compound . Along the path was a ditch 0 @.@ 5 metres wide and 0 @.@ 75 metres deep which then became a steep decline . At the outer end of the lower part , and connected to the 9th tower with a low wall , is tower 10 , which the Turks added to act as a lower artillery tower . It controlled passage along the Danube and guarded the entrance to the harbour , which was probably situated between towers 5 and 10 . There are remains connected to tower 8 which probably formed a larger whole with it , but the lower part did not otherwise contain buildings . 
