Here's an idea for a new achievement (do the devs read this board?): Finishing survival mode on a number of different maps (4, 5, and 6?).
I mostly only play the basic map, but recently have been spending some time on the others. Things like Factory and Ruins were pretty easy to suss out, but the Courtyard is pretty tough.
I've worked out a strategy that's doing pretty good on the Courtyard at level 300, and will probably work for up to about 500 or so, but I reckon Survival mode is going to be hell!