NES Classics: Zelda II: The Adventure of Link

You can bypass a forced encounter if you enter the tile while it’s being occupied by a random battle monster. Instead of fighting through the forced encounter you will fight a random battle. After the battle, you can simply step off the tile and continue.

If you need to get through a locked door in a palace but you don't have a key you can use the spell of Fairy and then fly through the gate using this glitch. Handy if you don't know where a key is or want to save time.

In the GBA version, press the start button and then simultaneously press the select button and up on the D-pad to skip to a menu with the options to "continue" or "save." This is the same menu that appears when you run out of lives. Selecting "save" records your progress and returns you to the title screen. Please be aware that selecting "continue" restarts you with full lives from Zelda's castle. There is no way to cancel this menu once it is brought up.

There is a spell hidden in each town. Their magic cost depends on your Magic level. Shield reduces damage by 50%; Jump doubles your jump; Life restores 3 hearts; Fairy allows you to fly and pass through keyholds; Fire lets you shoot fireballs from your sword; Reflect lets you block fire and reflect magic waves; Spell turns all monsters into blobs and unlocks the Magic Key at the end of Kasuto; Thunder kills everything on the screen.

If you fall off the side of the screen when you get hit, you won't die, even if your HP is at 0. Stand very close to the edge of the screen, and attack from there. You can use this to level up without the risk of losing your EXP to a gameover. Especially at screens with jumping fish.