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Keepers Comeback Campaign

@ComGaunt: In that case you are missing the level keys. Similiarly to the original Dungeon Keeper 2, the bonus levels are unlocked by finding certain artefacts in the previous levels. If you do, a message will appear telling you that you gained access to a hidden realm.

Hint: Take a good look in Level 6 and Level 14. ;)
More specific descriptions can be found in other comments.

I have finished the campaign including level 18 but the two bonus levels (Caradrico and Freetown) are not unlocked. Am I missing something?

@Blink: The Underhill only shoots banshees at enemy cores, however, there is no enemy core in this level. The underhils have different effects, which only activate in levels 19 and 20 as reward for those who are thorough. In level 9, you only have to KO the lord of the land, who is in the north (it's a atronghold with two entrances). The underhill and the magical girls are optional.
The gateways can be stopped as shown in level 2, although admittely this level makes it difficult due to them being out of the way and waves spawning quickly. If possible, try to stick on one side of the map until you deactivated the gatway. Alternatively you can put earth on the small tunnels leading to the gateways, but that won't stop the gateway from spawning more units, which can lead to performance issues if kept too long.

I must be missing something screamingly obvious! What do you do with the Underhill to launch balls at the enemy? I'm stuck on level 9. I have explored everywhere but there are two gates at the top that just poor enemies, so It must be the underhill thing on the right that I claimed, but I can't work out how to activate it. Thanks.

That worked, thank you.

@BoogieMan: It has been some time, but I do recall it's possible to get him out by possessing him through the menu. So rather than using the spell on him, you click on him in the menu. This does get tricky with the one challenge, where two units are hidden though.

What do I do in KCC11 with the possession door near to the dungeon heart? I cleared it of all enemies (no traps found) and rescued the Auger at the end. However, I can't get him out because it stays under fog of war and he won't leave through the door on his own even if I open it for him, and since it under fog I can't posses him to walk him out.

Congratulations. Truly well deserved.

@BBC: Glad to hear it! Hope you can enjoy the game without issues now. :)

hey padel its bbc no worries i did end up reinstallin the game and it worked out thanks for the help