I've just added a Regal Realms expansion to Ironfell (indie, hex-tile, MMORTS). There are ten new units and two new Keep and Castle realms.
The new realms use a brand new 'Policed' permission type; they are PVP, but the creator of the realm is invincible. This gives players a safe place that can't be spoiled by anyone else.
They have already been used as battle arenas, with the owner of the realm acting as referee. Players talking in the forums about Arena Battles they organized: Ironfell.freeforums.org
"Whilst engaging the majority of Gimley's forces in battle, Zathor sent a few lone soldiers to assault the princess, sweeping aside the catapult's like nobody's business and transforming the princess into a pool of blood."
Check out the new units here: Ironfell.com