What this code does is give you a "low" var that you can set to make a turf able to be flown over. Otherwise, flying lets you move over dense mobs and objs, but not dense turfs. I use the "low" var for bushes and outside walls (like the wall around an fortified village), but not mountains (too high) and the walls of a house (yes, you should be able to fly over a house, but the way the map is set up, this would put you inside the house.) You can use it however you want. Make all your dense turfs low if you want everything fly-over-able.
I'm not good at commenting code, but the Enter proc is the confusing part, so I did my best there. A couple of notes: one, this assumes that all mobs who will use the fly verb are dense, and that all turfs using the low var are dense. (If they're not, then you don't need to label them as low... anyone can move over them anyway!)
Note to anyone who uses this code: Type everything exactly as I've put it. Do not add anything that sets a mob's density to 0 when they take off. This is not necessary and makes the rest of the code pointless. This code works. If it doesn't, then YOU are the problem. Don't take credit for this code, but DO NOT MENTION ME ANYWHERE IN YOUR CREDITS OR I WILL SUE YOU FOR DEFAMATION OF CHARACTER. Do you think I want my name on your stupid game?