So starting with with the weakest of a faction, fallen, hive, vex, whatever, a fire team of six, no less than 3, must hold the line of a desperate hoard bent on the overrun of the tower. 30 seconds between waives for ammo synths and weapon swaps. The payout...well since everyone in the tower has been saved from a horrible demise... Again..., I would imagine the rewards would be great. But don't be fooled thousands of desperate thrall ain't no joke!