filling a hollow object?

Like for example, a 360 degree arch (circle) is automatically just a circle. I've tried using the vertex tool to change it to a solid object, but that just messes the whole object up. I guess I could try making it myself, using a circle arch to carve a normal square block. But I'm kinda afraid of using the carve tool anymore because of how much it messes up.

Yeahhh that was it. I figured that out about two minutes after I made this thread.