This is the Mob arena I use/used for my server. All required rooms (Lobby,spectator rooms) can be found in the central structure. The lowest floor with 4 exits (1 in each direction) is the arena starting point, giving the players 4 routes of escape to prepare for the mob invasion. The next floor up (room with small glass windows) in the spectator room. The spectators have minimal view but you can change that to your liking if you wish, I kept it how it is because I though it looked cool :) The final floor is the lobby room where the players who joined this arena can pick their classes.

NOTE: You can change the spectator and lobby rooms around if you fell it would work better but giving the spectators viewing holes of the arena I left them how it is.
Also the world download is in 7Zip format.
You also must have fire spread off in the world or world guard reegion if using it in a server as it will probably start burning before you create the arena.

Please comment if you find any flaws/glitches etc so I can fix them quickly.