Basically its one of the things Lineage 2 got correct. Its siege warfare.
Basically your clan/alliance holds the castle for 2 weeks and then at set times (set according to timezone), other clans can sign up to either defend or attack the castle. The defender also has the ability to buy defending NPCs, traps and reinforce the castle. Basically its all about who can destroy the attacking players flags or take over the castle holders controller crystal.
The castle holders get special things like the ability to set tax rates in their region, buy and sell special things and such. There's even larger castles which encompasses all the castles and regions in the area. These are very valuable to hold.
If it could be given other benefits in UO, this could be a great idea. People on the same side would not be able to attack eachother and it would be possible to destroy both the walls and doors to the castle.
This might even be doable in Trammel, where it would be a PvM siege and maybe give less benefits than Felucca held castle.
Also, it would be possible to buy certain resources in packages during the siege, to keep the sides going. These might be restockable vendors that each side can stock up before hand to allow quick re-entry into the battle.
Upon death you can resurrect at your flag or crystal, but there's a time penalty. Also, durability loss is lessened or removed entirely during the siege, this is necessary to keep the fighting properly intense.