[Coolboyman]

Yeah, Gruntilda's lair was the final part of the game being built.

Here's the order that the levels were actually first made:
1. Treasure Trove Cove (AKA Hammerhead Beach)
2. Gobi's Valley
3. Mad Monster Mansion
4. Mumbo's Mountain
5. Rusty Bucket Bay
6. Freezeezy Peak
7. Spiral Mountain
8. Bubblegloop Swamp
9. Click Clock Wood
10. Clanker's Cavern
11. Grunty's Lair

As far as I know, this is the order for Banjo-Tooie
1. Mayahem Temple
2. Glitter Gulch Mine
3. Jolly Roger's Lagoon
4. Witchyworld
5. Hailfire Peaks
6. Grunty Industires
7. Cloud Cuckooland
8. Caludron Keep
9. Isle 'O Hags
10. Spiral Mountain
11. Terrydactyland