Build a ramp down under the wall and back over, so It will form a valley-like shape. This is particularly difficult on this map without getting some mice stuck.
If you want to challenge the mice, try going up and over the wall instead. It is possible, but if you and the mice aren’t careful, you’ll be bouncing off it into possible doom. You can also conjure yourselves through the wall by building a cage and conjuring behind you, but make sure to build a bridge at the other side so that you don't fall to your death.