Reward: Sex Change Souvenir
Once you purchase the castle navigate yourself to the Regal Bed (this can be tricky) and sleep in it. You'll wake up all nice and refreshed until you are informed that your precious castle has been attacked by Bandits! Just outside of your bedroom you will encounter a small group of Bandits, kill them easily then proceed down the spiral staircase into the throne room.
In the throne room you will have to single handedly defend yourself from a few waves of Bandits, this was suprisingly easy, seeing as how they're trying to takeover your castle. After defeating the Bandits in the throne room you will have to make your way to the library to kill their leader.
Of course you get to the library you'll see that the Bandit Leader is not alone, there is a handful of Bandits with him, but you can easily slash them since it isn't a large room. Oddly enough the leader isn't the huge guy with the cape, but he should be the one on the stairs to your left. Kill them all and your whiny butler will quit, leaving you to investigate the tomb on your own.
It's worth noting that once in the tomb that you will encounter Bandits, but when you make your way to a room with prisoners walk up the the door ahead of you a pull the lever on your right. The lever on your left will open the doors and you'll have to fight a bunch of enemies at once. In the next area you will have to fight Hobbes down a few flights of stairs, there aren't all that many of them so there should be no challenge for a noble hero like yourself. Before you go down the last set of stairs you'll notive a chest below you to your right. You can vault down and open it, there's 1500 gold in there.
Next you will have to dive down another level, there is a dive spot near you if you look to your right while swimming. In the next room are more Hobbes, kill them then explore around a little, there are two chests in this area. The easiest thing to do for the switch in this room is to stand adjacent to the door behind the pillar and shoot the switch, quickly shoot it a second time before it resets, then whack it as it sits next to you. The door will open after you've completed this successfully.
In this next room the switch is timed again but it isn't hard, when it is glowing pink send a spell towards it like Blades, Shock, etc. When it is yellow shoot it with a ranged weapon. The switch will go pink, yellow, yellow, yellow before it opens the door.
Upon entering the next room you will hear an announcer who will inform you that you have this one chance in the game to change your sex. It is permanent and all that jazz. I'll give you a chance to think about it while you walk around the room and open the chests. Walk up to the potion and grab it to open the door. Drink it if you want, if not no big deal.