It may looks just a huge hole into the rock but in the past has been used as a weapon.

The Fougasse is a sort of mortar.  The hollow was filled with explosives and projectiles and then fired out.

Mostly used, at least here, in coastal defence. In this case to protect Fort Chambray.

Another famous fougasse in Gozo is located at Ramla Bay.

More info at wikipedia.

Sea view from the coast