That's a really good point! It's something that I found bothering me a bit too (the backpedalling heavy gameplay, due to the fast running zombie guys). I am currently fleshing out some enemy designs that encourage (or require even) chasing down and being very aggressive. One will be an egg laying type of enemy that will actively avoid the player and be a threat purely by laying eggs out of which lesser enemies hatch. And there's a few more I have in mind that I think will change the gameplay in this sense. You're definitely spot on with the level designs concerns too, as I'm working on a first set of campaign levels I'm finding that my current enemies don't hold up so well in tight space. Might actually end up doing campaign-only versions of them with tweaked ai's... Thanks for the awesome comment!