This was described on Day 121 here is the snip it from 121 and how it was put.
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<<External Training Grounds>>
Due to the vast increase in the number of members, I had already generated a vast space for the purpose of large-scale combat drills and other purposes. I had created nearly a [200 x 200 x 300] space for the purpose, though while we were gone it has been equipped far more efficiently. The rest of the room had been flattened, and rough terrain had been placed in certain areas. The reason I had created this room had been previously for the sake of bad weather, but now more intricate training scenarios can be implemented.