Press [Ctrl][Alt][C] and type in one of the following cheats:

show me the money       1.000 000 units Gold or treasure room filled up
feel the power          All monster are set to level 10
this is my church       All rooms accessible
i believe its magic     All spells available
fit the best            All rooms and traps accessible
now the rain has gone   Map complete
do not fear the reaper  Win the level

And if you have the latest patch installed:

ha ha thisaway ha ha thataway  Plus 100.000 mana


If you did everything right you will hear a sound of a door.

To start the game in a specific level start the game with

dk2.exe -abc

where abc can be one of the following level names:

level1
level2
level3
level4
level5
level6a
level6b
level7
level8
level9
level10
level11a
level11b
level11c
level12
level13
level14
level15a
level15b
level16
level17
level18
level19
level20
secret1
secret2
secret3
secret4
secret5