Sentence:  Construction on this massive fort was never completed. ... Only the foundations and a few tiers of stone were completed on the two <WSD>gorge</WSD> walls and the <WSD>gorge</WSD> bastion. At this point, a significant modification in design was made. ... In this way, the <WSD>gorge</WSD> of the fort was closed at minimum expense. 
Meaning of the word marked with <WSD> tags:  The entrance to an outwork, such as a bastion.