Replace lever with a horizontal coordinate trap in City of Skulls

[epyon]

For difficulty between City of Skulls and Abyssal Plains, the big thing is that you can enter and exit City of Skulls with no worry of anything if you want to. I think a simple solution here is instead of unleashing enemies with the lever, you simply have the pillars come down once you pass the hallfway point of the level (while also expanding the middle pillars so that you can't run through the top or bottom of them), much like how the Phobos Anomaly's trap works.