It's like playing the individual guys in an rts. I'll try to describe what happened last night:
Given a mission to take an enemy stronghold, get to the front and there's about 30 or 40 of us laying siege with trebuchets, ballistas, and catapults. Break through one wall of the keep, but their defence pushes back, and they turn that to offense and about 20 or 30 of their guys pour out. Or front gets slaughtered and we're pushed back all the way to one of our keeps, losing a stronghold in the process. Now we're on the defensive, and under siege. We push back and end up overtaking them, recapturing the lost stronghold and finally capturing the keep we initially were laying siege to.
This was all in first person/3rd person. Having a mount is crucial as the distance between keeps is huge. Best part though was running with so many other players over the hills. Freaking intense.