Sira 'Island' (it's actually connected to Crater by a causeway) is an 'impregnable fort within an impregnable fort'. The whole Crater district of Aden is itself protected from the mainland by steep cliffs and the fort on Sira Island could very easily defend the approaches from the seaward side. The old Turkish fort on top of the island is sadly classed as a military site and is thus off-limits. A great shame as the view from up there would be terrific.