I'm going to upload the demo in a few weeks. And if you're interested in designing a level then i'll send you a template level with the character and all his movements and guns etc. and I'll chuck in a few graphics.
Don't bother about doing any graphics yourself because I have some people doing that, unless of course you're really good and feel you must, well then go for it as long as it fits in with the other graphics. But the main idea is to design a fun level and I can fill in the rest.
In terms of my custom movement there are no slopes or ladders, only springs. I couldn't figure out how to make the ladders and I figure I could get it done with a spring. If you know how to program the ladders, then feel free to do it for you level. Your level will be credited with your name or alias.
80% of levels are 800x600 and I work to a 40x40 grid, so boxes and blocks are all this size. Some levels are 1600x600 and my engine is capable of scrolling for those levels.