What made you decide that Robinson Crusoe put gun holes in the wall that protects his castle?

I don't know who answered the original question, but you can find this information in the text.

"So that I had now a double wall; and my outer wall was thickened with

pieces of timber, old cables, and everything I could think of, to make it

strong, having in it seven little holes, about as big as I might put my arm out

at. In the inside of this I thickened my wall to above often feet thick, with

continual bringing earth out of my cave, and laying it at the foot of the wall,

and walking upon it; and through the seven holes I contrived to plant the

muskets, of which I took notice that I got seven on shore out of the ship."