How to hide parts of an object?

Hi there,

I’ve got a flat platform as a floor for my map, that has a cylindrical wall, so looks like the attached.

Outside of this is a pretty nice outdoors environment as a backdrop.

The problem is that as the player gets to the end of the wall they can look over and see the edges of the square floor of the map protruding outwards and it looks pretty horrible, is there anyway of hiding the edges of the floor that are outside of circular wall?