Gruntilda's Lair serves as a hub world. You'll have to climb your way up through Note Doors and placing jiggies into the puzzles. I liked the soundtrack here, it sometimes get stuck in my head.
I liked the lair. The music is pretty catchy and I loved to explore the place. Of course, I hated the water areas in it. I also didn't like falling in the lava all the time :-D.
Did you guys ever have trouble getting around? I remember when I was first playing the game, I would get near the Rusty Bucket Bay area and the Mad Monster Mansion puzzle thing, and I would get confused on what to do. And I had trouple finding the Click Clock Wood puzzle thing.
I remember when I finally got the ice key, I kept trying to go to the top of the lair where the machine that transforms Grunty is, and try to unlock those two doors on the side. Never worked.
Ice Key proved to be a useless collectible here. I always wondered where those two mysterious doors led to. Maybe some levels you don't even know? Maybe Fungus Forest? Mt. Fire Eyes?
I also didn't know until like 3 years ago that you could actually go back to the Lair in your transformed state. That's how I finally got the Jiggies on top of the Mountain entrance and on top of the Wood area.