Holdout Keep is a six player map that was used for the Commander's Challenge in Command & Conquer: Red Alert 3: Uprising. Its description is featured with water and coastal lands around a small island in the middle of the map.
There is water surrounded by coastal lands and a small island in the middle of the map. That island has a castle wall that surrounds the player's base just like the Commander's Challenge mission, Tesla's Castle. The final mission in the Commander's Challenge, Future Warfare uses this map and takes place in the Soviet Union.