I think you are at the top of the furthest right screen of the map if you saw the door with a keyhole. From the top ledges, probably in the third ledge (i can't remember), when you jump or drop down to the second row of ledge, there is an entrance (black hole). Once you defeated the enemies inside, you will find an object or key to open that locked door from the top middle ledge.
Hope this helps, but you are not missing much. As the game is "to be continued" until the next update comes out.
