The main plot in my current campaign (as the PCs have just discovered) involves a vampire goblin named Lord Hex who is trying to gain access to an evil tower known as the Goblin tower. The tower is rumored to bestow fiendish powers and allies to he who sits on the throne (Half Fiend template with some Fiendish cohorts thrown in).

If Lord Hex gets into the tower he will forge a goblin/devil army and subjogate the surrounding goody goodies.

The players know he has tried to get in and has failed. They are going to see an old sage (there is always an old sage) to find out more about he tower and how it was locked. So they can figure out Lord Hex's next move.
Here is where I am stuck. How do I lock this tower so that the bad guy needs to seek something (key, object person) that the players can try and go after to stop him.

They party (of 8 PCs) is about 3rd lvl avg and Lord Hex is probably ECl 12, so I don't want them dancing just yet.

Any advice???