How do I get up the ladder in the dungeon?

Well, the answer to this one is very easy.  You don't.
You'll have to find another way out of the dungeon.

There really isn't much of a way to explain this without giving you an exact
answer.  If you don't want one, my only advice is that you try everything and
take note of what works and what doesn't.

Start by having Magician "spell" the skeleton.  A bone falls on the ground.

Then have Magician cast a spell on the bone.  You will get a flute.

Have you ever seen a snake charmer?

Then you should try charming the snake in the lower corner of the dungeon.