I really thought, as Eren was reaching to open the basement door, how fun it will be if the key didn't actually open the lock, then it freaking happened! That alone made the 6 year wait worth it
They even made sure to show them walking around towards the prize rather than directly cut to when they're at the basement door, that's a nice touch.
Maybe we'll get some long overdue answers about this world next episode, I could've checked out the source material years ago but where's the fun in that?