This is basically what it says on the tin.
Its a piston based arena that was designed to allow battle between all of your little friends. A lever simultaneously opens all of the 'gates' and raises bridges and blocks to change the topography of the arena.

I think its nicer than most arenas I've seen and its also easier to operate and its pretty fair so the teams are quite even.
This is the same arena that is in my main house so if you like more stuff like this then go check that out :)

Obviously you need the clay soldiers mod to use this....