you havent said where the hotel is

I'm guessing the Atlantic Hotel Banjul is in Banjul Gambia.

This section of the Gambian coast is very vulnerable to erosion as a result of storm damage - they are constantly taking measures to try and prevent further erosion. Dumping large amounts of large boulders at the edge of the shore line is a very common way of trying to prevent further erosion - they did it where I live on the east coast of Scotland as part of a bigger scheme of coastal protection and it was a very effective holding measure. Depending on when you were last in Banjul I wouldn't be surprised to find that they have since taken this sort of action.

SM