Commander Keen Episode IV: Secret of the Oracle

Bonus Level
To find the bonus level, enter the Pyramid of the Moons. Take the yellow diamond and proceed downward until you see the second yellow keyhole. Use the diamond on the second yellow keyhole. Here you will find several yellow worms. Assemble 12 of them in one place and they'll turn into a giant foot. Jump on the foot to enter the secret pyramid.

Debug Mode
Press B + A + T to enter debug mode. You'll receive shots, gems, and an extra life. You can also access the following cheats by pressing the indicated key.

B - Set border color (1-15)
C - Number of active/inactive objects in the level
D - Record a demo
E - End or finish the current level
G - God mode (you can't die)
I - Free items (add 3000 points and 99 shots)
J - Jump mode (you can fly anywhere)
M - Memory usage display
N - No clipping (fall through ground)
S - Slow motion
T - Sprite test display
V - Add 0-8 VBLs
W - Warp to any level
Y - Reveal hidden areas (shown in normal color)

Debug Mode - Alternate
If the code above doesn't work, press Shift + A + 2 at the same time. You'll receive shots, gems, and an extra life. You can also access the following cheats by pressing F10 and the indicated key simultaneously (eg. F10 + G = God mode).

B - Set border color (1-15)
C - Number of active/inactive objects in the level
D - Record a demo
E - End or finish the current level
G - God mode (you can't die)
I - Free items (add 3000 points and 99 shots)
J - Jump mode (you can fly anywhere)
M - Memory usage display
N - No clipping (fall through ground)
S - Slow motion
T - Sprite test display
V - Add 0-8 VBLs
W - Warp to any level
Y - Reveal hidden areas (shown in normal color)

Power-Ups
For 99 shots, an extra life, and all keys, hold T + A + B while playing.