Tip: Switch palace switches normally disappear if you replay the level. However, if you insert them as direct Map16 and use the hex-edit at $00EEB2 in the ROM map, you can replay the level without a problem.
This is my level that I've almost finished. The gimmick is that Mario (or Luigi) traverses the same area at least twice: both during the day and during the night. At night, everything freezes over (hence the level name). Turn blocks and block bridges freeze solid; water becomes deadly cold; the ground freezes and expands in a few places; everything is slippery. In order to get the normal exit, you need to play through the area at least twice to get the necessary item (there's a little shortcut so you don't need to go through a third time.) I've uploaded an IPS patch, so some feedback before I submit the final file would be nice.