I like Shadows idea but he is right with the AI. Creating an intelligent and challenging horde of enemies is not so easy. The rest of the game can be made quite simple, graphics, collisions, moving through the world and stuff like that is not really hard. Take my little test here as an example, tiles and walking through the world is easy. The hard part is making enemies that do more then simply walk into your direction.
http://www.mediafire.com/?xbrkb6k1hcn4ff5
WASD to walk T for collision info. No enemies included right now.
EDIT:
http://www.mediafire.com/?ij7n6nuncdjdgxj
Same as above with one stupid enemy. Hide behind a wall and he can't get you.
