Ywkls, interesting colours. again the dungeon walls especially the corners look to have a weird perspective. seems to be a lot going on puzzle wise on this screen - crystals, hammer pegs, blocked chest, locked door, telepathy stone - its not immediately obvious what is supposed to happen, especially for what looks like a dungeon entrance - can the player actually do anything or go anywhere?
Not at the point that in the game whenever this can be first accessed. Basically, this is going to be a final dungeon where you can go inside as soon as you like, but progress is blocked by the lack of major items acquired in other dungeons. In order to actually explore, you'd need to get the flippers and the hammer and then come back.
As for the chest, the clue on how to reach it is inscribed on that slab. This tells you how to hit the switches, which in turn lower the pegs. What's in the box? Not a key... no, that's going to be part of a sidequest you can initiate outside of the dungeon. Which would provide a motive for stepping in here before you can actually defeat the dungeon.