Nördlingen
Nördlingen sits in the middle of a meteor crater.  The town is round with a nearly intact medieval wall, and the church sits smack in the center.  You can climb the tower for terrific views all the way around.  You can see how circular the original town was in the sea of red roofs before you, and you can see the rim of the crater off in the distance- the crater is about 20 km in diameter.