In one of the events which apparently occurs at random while you are in the arena, a giant turret will sling arrows at you till you destroy it. In the other reward statues of Roman deities will appear out of nowhere and add bonuses to your health or focus abilities.

If the free content does not excite you, the new Coliseum Pack that has been released for $3.99 will add two new arenas and two new player skins. The new arenas are called Henge and Ascension.