Deadly Towers

Battle with Rubas:
Enter EF1KDK.KTX, 54DK1P4WC, or .AZODKNXTJ as a password.
Information in this section was contributed by bking.

Extra items:
Enter FE1K10.TDK as a password.

Enter GDXHDPMM10 as a password to start with all attack and defense items, Hyper Boots, and 5 Bells.
Information in this section was contributed by mixup2001111.

Super passwords:
Intentionally die immediately after starting a new game. Change the first two letters of the password displayed to "EF" or "FE", then re-enter the revised password. Your character will have 102 hit points when game play resumes.

Enter FEZN3I6P.Q as a password for 222 hit points, all the armor, and six out of seven Bells.
Information in this section was contributed by Levi.

Hint: Upgrades:
In towers, or on your way up to the entrance of towers, walk up and down each floor, and in the corners of each room. Most towers have a secret room with a upgrade sword/shield or a figurine, and some have a secret "parallel" version of the tower, where you must look around for an upgrade and search for the exit.
Information in this section was contributed by Chris Hardewig.