Hello, to be able to upgrade higher than rank 4 you Maze rating has to increase, the higher your rating will be the higher your Maze will be rated. I dont know the excat numbers, but my maze has 600+ rating and is on Nightmare. The higher your maze got rated the higher you can upgrade the rank on your Defenders for example with Nightmare you can upgrade up to rank 18. It starts at Easy which everyone start withs to Medium, High, Very High and Nightmare. When your Maze gets up or downranked you will get an Mail about that. Increaesing the rating of your Maze happens when Players try to plunder your Maze and die but sometimes randomly your Maze will also get up or downranked even tho nobody tried to plunder it. Hope that helps you and clarified your Questions.
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