-Level 5 -
This is probably the trickiest dungeon to find, so I'll give some hints.
-The first entrance has to do with diving around an island. If you look near the NE part of the overworld (near where you enter level 2), you find a portal to the past. The house there in the past tells you exactly where to dive.
-The second enterance is in the West Shrine
-The third entrance is "atop the highest mountain"
In the 1st and 2nd entrances, you should have a new weapon and (I think a key) before you continue to the next one. (Hopefully I'm not spoiling too much). Overall, the dungeon is really clever I think. Take note of stuff like odd colored tiles and torches. Meet in the middle where they would meet if they would cross, and dive and look for keys. That's the best hint I can give. Eyes also don't like a certain weapon