If you build on the ground level, then make sure nothing can tunnel underneath. I used railroad ties to build my aviaries. I completely buried one tie, and then set another tie on top of that one, for all sides. Then I piled rocks all around the base. Nothing has ever tried to dig under.
If you are going with a raised pen, then I would get it off the ground. High enough that nothing can fool with it at quail height or lurk and crawl underneath. So you might want to get it up 3 or 4 feet. You can wire off the bottom, wrapping wire around the legs all the way to the ground and bury the wire as well a foot down. If you don't want to do this approach, you can use catch pans, like parrot cages have, or some sort of baffle under the wire of the pen floor so that nothing can reach up and grab toes and claws in the night. Make sure to secure the catch pan well so nothing can fiddle with it in the night.
You also don't want critters being able to look into the pen or directly at your quail in the night. Will cause panic and flushing to the ceiling.
