Go down the hallway in to the room. A cut scene will begin. Red robe people attack you after the scene. Have your minions kill them.
Go through the door to the left. Keep going through other door.
All you have to do here is kill the robed people until Sir Willaim appears.
Once he appears just attack him. When he loses half his life the little coward runs.
Go through the door that Sir William opened. There will be red and brown summoning pits. There is a door to the right. Exit this room through the door.
Follow the hallway after the door in to a room. Oh joy more succubuses. Two of them in fact. Have your minions jump on them when they land. Hit them with your weapon as well or Flamethrower.
A door opens when the succubus die. Go through the door. Following the new hallway you will come to a broken red wheel. Velvet will talk to you through the door. Follow hallway that is to the left of her door.
At the end of the hallway will be a door. Enter the next room through the door. Sir William and a bunch of the red robe people will attack.
Have your minions attack the red robes first. Sir William will teleport away from the center of the room every so often.
Wait for him to return to the center and attack him. Repeat this until he is dead. More red robed people will show up during the fight. When you kill Sir William, a cut scene will begin. After the scene walk up to Sir William and hit him with your weapon.
Sir William will leave the rod piece. Have a minion grab it and run it back to the wheel.
Go back to the wheel. Have your minions turn the wheel to open the door.
Enter the room to meet Velvet. Rose will appear and demand you choose one of the sisters. Uncorrupt Choose Rose by turning the wheel.
Corrupt Choose Velvet by carrying the bed.
You will be back in the Tower. Rose will have some upgrades for you brown and blue minions. Velvet will have upgrades for you red and green minions. When done upgrading minions/tower/equipment, time to go back to Evernight Forest and kill Oberon.