Here's how I got to level 50 (and still going) on Drylands (with only the original 4 weapons):
Figure out where the north-south run is, and build two walls of 3 machine gun turrets each, extending out from the top and bottom edges, to the immediate right of the north-south run. The west-east runners on both paths will run straight into these, and you catch the much less frequent north-south runners with both nests.
You're going to want to max out the guns and add a chemical(?) turret to slow things down. I put these chemical turrets to the left of the north-south run, one row in from the edge so that the runners have to run around it.
After that, just enjoy the mayhem until you have enough cash to buy teslas. Start putting a tesla behind each of your walls of guns as soon as you can. You're going to want to have one covering the top west-east run by level 18, as this is a tough combination of P-47s and other runners/riders. You'll get the same formation on the lower west-east run immediately afterwards, but you should have enough to put a tesla down there by the time you clear the previous level.
You're good now for quite a few levels, but you've got a couple of things to balance. You'll want to build more teslas behind your guns (2 or 3 in each nest is plenty for now), but by level 34 you're going to need to have started your gauntlet because that's when the first serious bums rush of runners arrives. Without a gauntlet, these little buggers will run right past everything and you'll lose double-digit lives.
For my gauntlet, I figured that I need serious firepower along the top and bottom edges to catch both the runners and the flyers, and then a simple rat run to funnel the runners to these edges for summary electrocution. To achieve this, I extended the existing walls of guns to the middle, leaving a one space gap to allow the runners through, then extended a corridor north and behind the teslas that are at the top edge. Then I put a couple of teslas around the corner and turned the corridor south so that the runners have to go past the teslas at the bottom edge.
Be careful when building your gauntlet because, while you want teslas at the first turn in the corridor at the top edge, you can't ignore the west-east run at the bottom where you can get overrun by 'copters and P-47s if you don't add some down here too. You'll also want to add chemical turrets in strategic locations to slow everyone down. You've all played the game so you know what I mean.
Anyway, this got me to level 50. I will continue to build my gauntlet winding east towards the exits and packing the top and bottom loops with teslas. I will then start to build a wall of teslas running top to bottom along the left side of the north-south run (which at this point is completely open) and then extend teslas just in from the top and bottom edges, back towards the entrances. I figure I can pack these three deep with teslas, starting at the intersections of the north-south and west-east runs and building backwards from there. The north-south wall takes care of all runners and north-south flyers, while the west-east walls do the same for all runners and west-east flyers.
I'll let you know how far I get.
ETA: Running your gauntlet north-south has the added advantage of separating the runners from the flyers, meaning they have less chance to overrun your defenses.