Bridges can’t go through mountains, as the player cannot walk on those kinds of sprites. Other wise it would get really weird. It’s hard to explain, but if you send me the link i can fix it and show you that way.
Just save this oer your file (Well, save it as another one first to check and make sure it's okay and THEN save it over yours.) I also fixed the ladders for you
~The first level is flipping hot, yo; that's nice designing skills.
~So, the issue was that you put a lot of the mountain sprites down, but then covered them up with grass. It doesn't work that way. You have to actually delete them, otherwise the player will still register it and be unable to pass.
~Another issue you were having was placing ladders and bridges OVER the sprites. It doesn't work like that either for the same reason.
How I fixed it:
I deleted the mountains and just placed the bridges over the default ground. Then I placed the mountain sprites AROUND the bridge, but not where it would interfere with the player (AKA, WALKING on the texture). For the ladders, I also added the cobblestone tiles under "Ground" to make it LOOK like the player was climbing over the mountain, but it was actually just like walking straight across.
Check out the other games on here, and you can see a lot of useful tricks. I learned this one from the default game!