Dragon's Den in the Mirror Tower?
 

When I enter the Mirror Tower in Elon I can go downstairs to a dungeon.
It's just a small room but you can see a Dragon's Den and an exit behind the wall. How can I reach it?

I thought I would have access after deafeating Zigaldis but no...

Thanks!
starstuff

Its accessible via another underground entrance - from where your boat appears initially on Elon, sail a bit to the left.