The Secret Dungeon:

     Best  Place  to  Get Invulnerbility and Max XP. There is a hidden
four-floored  dungeon  in  this  game. To go there, you need a Dark or
Light  Magic  Spell Ring. Go to the beach where you start the game, go
to  the  left  and  you'll  find a door that is locked. Cast a Dark or
Light  Spell  on  the  door,  and it should open. Inside this dungeon,
you'll  find  some pretty hard monsters. They can kill you in one hit,
so  be very careful. There are Lvl 5 Magical Stones of all kind, Magic
Rings  (in  the 3rd floor, there is a bird that give them infinitely),
Special  Rings  (including the Mirage Edge, that makes you hit 4 times
instead  of one), and the Eternal Sword (you have to kill a monster in
the  last  floor  to get it). This is also a great way to gain tons of
XP.  The  best way is to kill the big birds in 2nd floor. The best way
to  kill these birds is to get Dragon Fangs by killing the white crabs
on the 4th floor. Every time you kill them, they'll drop a Dragon Fang
that  will  raise  your  stats by one. Keep using these until all your
stats reach 255, and you'll be strong enough to never die.