That dungeon is the only missable content, but you should trust me when I say you should avoid the optional content altogether in this quest as it will only make you very angry. You can come back and check the stuff out with level 4 cheat later once you beat the game and are given the quest password.I started this quest and looks very fun. I completed levels 1 and 2.
I discovered the Ultimate Challenge, but I have more than 3 hearts. Have I to start the game from scratch to complete the dungeon? = ( And if there are more missables dungeons, I'd like to know it... I hate missables in games =(
By optional content, I mean:
-Ultimate Test
-Grayzone
-Pitfall (although this one is actually pretty easy)
-Veni Vidi Vici
-Moosh's Ways
-Level 7 Hardmode





