Stormin' The Castle: Secret area:

When you get to the fourth area when you finish Angelica, there is a secret key near the cabins in the front. When you go to the front, you will see the key. Damage the bars to get the key, then go to the back of the arena. Look up to find the gate with the keyhole. Get up there and go to the gate. Go inside to find a lot of Reptar Funny Money.

Thanks to RamirezXsJ