How to enter the Mirror tower?
 

The door says you need to be on the magic council, or something like that, so I thought getting one of those diplomas would do the trick, but nothing happened. I've probably missed a hint hidden in the text (which, for the most part, I don't read)?

Any help?

You need to complete a specific storyline here. First, free the elf king. Then he will give you the task to free the elf queen. After this is done, talk to the queen about the ring. She'll send you to the king, and the king will give you the ring needed to enter the tower.

Ah, okay, I should have suspected it had something to do with the occupied castle, as that is the only quest I haven't done yet (have already freed the elf queen). Thanks.

BTW, how does Zilgadis compare to K'Tahu, who for me was the toughest boss so far? The turtle and frog were both a breeze, and the driller presented some challenge (i.e. I had to reload due to the my shamans being buried under the rubble).

Zilgadis is easier, provided you do the battle properly.

Use stone skin on your 2 tanks that are near his right and left hands and use your remaining units to drop the towers he spawns. Some magic resistance is also needed here, because his mass attacks is purely magical.

In my opinion, K'tahu is much harder with all those summonings he calls all the time.

Zilgadis was easier than driller for me, did it no loss at first run in 8 rounds

I lost some dragons, but I didn't feel like reloading, because this was the last quest before the final fight. I guess for no loss I could have kept attacking with my royal snakes and destroying the summoned gremlin towers with my other troops.

Interesting, when I fought driller with a Warrior, he failed to get an attack off on me. A few retaliations that didn't do enough damage to kill my dragons, and he kept running backwards. His biggest attack was to knock down some stalactites... and then he died. Zilgadis was tougher... he actually fought back! I didn't have any no retaliation units though, which likely would have made him easier.