It was a great experience, glad I played it!
On the hidden areas thing, on the very first level there was a wall to the left, so I tried pause-jumping up there and I saw there was some characters moving on the top-left corner just outside your vision, so I assumed it was a hidden area. I couldn't get up there so I assumed I didn't got the trick to get there yet and moved on to the rest of the game. Glad I didn't miss anything, but it sure would have been very cool if there was a easter egg or something there though.