We had a cardboard castle party once... a bring your own boxes and duct tape type affair... great fun.
However, I highly recommend against letting a giant heap of wet cardboard sit on your lawn for any period of time. If you do:
1) Do not expect it to dry out unless you are in full sun and have low humidity.
2) Remember that wet cardboard will be heavier than dry carboard
3) Bring a pickup and a pitchfork.

This is one of the first books I read to my daughter. You can only handle so much Goodnight Moon, you know?