Well, I'm cutting it close, but here's an entry (an extra one  - not for judging since I pick the themes) for the current art challenge called PD Quick.

I kind of punted and picked Dracula as my public domain character since he's so popular. There was so much more to choose from, so feel like I'm cheating a tad. But it's free, so, eh.

The crypt of Dracula has the bloodsucker himself, bats, flashing lightning, a demon, flames, a skeleton and some bones that can be positioned for throwing. You also get walls, ceiling and floors with plenty of color variations.

The color palette chosen is limited NES colors - shades of magenta and black/grey only. I think it makes it pop, but your mileage may vary. If you choose to use an NES style you have some more colors to work with if you're combining assets. The graphics go as low as 8x8 (for projectiles) but really end up 16x16 or 16x32. Walls work best in groups of 32 so it leaves plenty of room for characters to walk down a corridor. If you care about such things keep in mind the size of viewing area and the total number of things moving around on screen, especially horizontally, for true NES limitations. If you go more modern then everything here is fairly small, so you can fit more on screen.