This is the underground room that we use for Mob Arena on Wrath Minecraft. I built it in just under a day so it's not the greatest thing I've ever made but I still think it's pretty cool. It works as intended most of the time but occasionally people get knocked up into the spectator area by creeper explosions so watch out. I definitely put form over function with this build, but it works well enough 99% of the time.

Also, this is one of my few builds where I forgot to check how it looks with a default texture pack. In my opinion the build combines some of the ugliest default blocks. Glass, iron bars, and brick are not a great combination. As a result, it is best viewed with Doku, Ovo's, or a similar texture pack. This is just my opinion though, most people say it looks fine with default.

The building is 100% underground, it has no exterior, nor does it have entrances or exits. The entire structure is contained within a stone brick cuboid measuring 51(L) x45(W) x 26(H).