This is Iaeyan's Fortress. It's kind of my most used signature piece. This version of it is designed for PvP, but you could potentially make a Monster Arena out of it. If you need entrances, I suggest punching holes in the sides where the dyed clay dips down.

I have a Nether-themed variant of this arena. I could post one of those if you want.

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For a good view, click that little Schemagic tab and go for a joyride!

Additional Notes

The schematic pastes from the blue side, at the right middle.