Haha, forget my tower defence analogy, this has basically turned into the D&D version of king of the hill
Suggestions for ruleset:
- 2d4 days between seiges
- start level 10, with level up after every seige
- gestalt, with retraining allowed only for one member on the defending team to take the Dungeon Lord PrC
- leadership
... i) you get max followers at the start of each seige, determined by your leadership score as normal
... ii) your cohort level is determined as if your leadership score = character level (taking extra feats or the Dungeon Lord PrC can raise this)