"Many daredevils were claimed by this mysterious fortress.
Its magical properties cause adventurers to warp between two borders, making escape impossible.
Can you manage to do it?"
This sandbox is quite experimental. The main gimmick is screen-wrap, similar to what you saw in Mario Bros. arcades. Walk up to the right edge, you end up on the left edge. Based on that mechanic are simple puzzles and platforming challenges. The actual screen-wrapping is done with two repeatable command blocks, as they change player's X axis. Players have to be moved 16 pixels away from these command blocks, so there's a one-tile border to mask that effect. If anyone were to replicate the effect, I'd recommend using farther camera distances. Just so you have more to work with.
This isn't the most robust thing in the world. If you manage to jump above the ceiling while screen-wrapping, you'll be teleported to the top of that wall. It's pretty fun and figuring that out, I felt like a speedrunner. So I left most of those spots in, now it's up to you to figure them out. 
Ambience is an ominous silence, broken only by sounds of your footsteps and jumps. I was looking for a fitting music track, but no luck so far. There are no deadly obstacles in the level, your biggest enemy will prove to be patience.
The sandbox took about 5 hours to make. 2 hours were spent messing around with the block indicator sprite and footsteps CP, so it's more like 3,5 hours on a good day. I'm glad that I made it, it was fun. Hope you enjoy it 
Screenshots:
Spoiler Playthrough:
Spoiler
