Some years ago someone realised a beautiful graphic work about the walls of a Roman arena. A trapdoor was added as special entrance for animals and gladiators. These walls look almost real, with a 3D effect, even if they resemble more the external of an arena than the inside walls. Through the Brabantini Yahoo group, a pdf file with these image was available to download. You could print it on paper, cut it to the wanted shape, glue it to card. Then your arena for miniatures was ready.