A: Monster Bunkers are developed primarily to increase your yard's defense with the use of your own hatched monsters. Once you build them you may fill it with monsters that you think will best defend your base from wild monsters and other players. You may find different combinations of monsters that is best use to fill those bunkers on the BM forums. BUT, the number of monsters that you may fill in the bunkers depends on the Monster Bunkers level capacity.
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Hope I helped you!