Where are levels 28, 29, 30?

Hint 1 of 7:

Those are the secret levels.

Hint 2 of 7:

They are well hidden.

Hint 3 of 7:

Look for secret rooms with exit doors in them.

Hint 4 of 7:

Specifically, look on levels 10, 21, and 24.

Hint 5 of 7:

The secret exits are all near the red doors.

Hint 6 of 7:

If you need help look on AOL or CIS's Interplay and games areas.