If you can't find another game you could try different mods for Warband. I know it's the same thing but... hey, mods can change things around a bit.
I would suggest multiplayer siege but siege in MP has some pretty major flaws that I wish they would remedy... I don't know of any mos that have fixed it either.
I'm trying to think of a game, any genre, that does this as well... but ehhh... nope. There's War of the Roses, but that's still in early development. There's Crush the Castle but that's not likely the kind of siege you are looking for.
Uhhh, yeah, aside from the games already mentioned I can't say I've ever seen anyone try to pull this off. The only stronghold I've played was the original, but I've heard Crusader is the best one. (I've played Stronghold 2 but... pfffttfpfpfff)
There are also various tower defense games that could kinda count as sieges...
Actually, a game did just come to mind.
Ace of Spades is a voxel based shooter wherein you have the ability to construct blocks, much like in Minecraft, so you can create bunkers and bridges and foxholes and all that good stuff to protect your flag from being captured by the enemy. I have not actually played it recently so I'm not sure what kind of changes have been made. [EDIT: While there are no catapults, grenades can destroy blocks, and even sustained fire can destroy blocks. I imagine some kind of mortars may be implemented in the future sometimes as well, which would be cool especially if it was something the player had to use manually instead of calling in god-mortars.]
In a similar vein there was Combat Construction for Tribes 2. You would start out in 'purebuild' mode where you could fly around constructing bunkers, deploying turrets, creating hangars and bases, all using beams and 'pads' (which were special beams that filled spaces in between beams, creating floors and walls). Afterwards, purebuild was disabled, and the match started. You would not have jetpacks but would instead use the constructions to move around and fight the enemy. Weapons were primarily projectile and bullet based and dealt a lot of damage, especially grenades which fired shrapnel. Vehicles handled pretty neat as well, helicopters could not fly forwards but moved forwards when the nose was tilted down, for example, and jets could not strafe left and right, but had to roll and then pitch upwards to turn. It was really satisfying and I wish something like there were actually made today instead of all this voxel-based nonsense.
A New Zero is heading for the voxel based construction, but it will also feature stress-based destruction as well. So buildings can collapse if you take out the support structures, for instance. Development is relatively slow and the current playable version seems nothing like what the newest videos have demonstrated.
Source Forts mod for Half-Life 2 involves placing blocks and wall-type things in order to prevent your enemy from reaching your flage. The construction is a lot more fiddly, but I haven't played it in years so I don't know if anyone still plays or if it's been improved or what.
[EDIT: Might as well throw in King Arthur's Gold as well, though I've not played it myself.]