1. Door with archers and crap. Break it!
2. On left hand side lotsa archers, run right!
3. Attack mobs on right. Stay away from left so you don't get pelted.
4. Go strait. Beat swarm.
5. Around corner archer wall, try to get a lot of nukers to AoE them all and run back out of range, rinse repeat.
6. Now you see a door, which is behind the was undead archers.
7. To the left a throng comes outa the wall. Kill that!
8. Go in door.
9. Another Swarm.
10. Raidboss is in there, hit it, and preferably kill it.
Grats for you!
The seige is best done with 4-5 groups of lvl 65+ (or so it seems). Unless you expect a lot of other clan interference, then it is best done with more than they got!
The hardest part of devistated seiges is bored clans trying to screw you up.