I was hoping to find strategy hints here ... with little luck. So I'll make a start with mine:
Which towers?
Land units are easier to keep at bay than air. You can deal with land unit with rockets and rockets also help against air, so rockets are my main weapon against land units (they are also pretty strong against land units, I think the game is a bit unbalanced there).
SAMS are the main defense against air.
Gun towers are required at the start of a game against land units. They get swapped out for rockets starting from wave 30 or so.
Slow towers are essential to both air and land defenses.
Teleports are required to get perfect games and help a lot in all circumstances.
Tower Placement
Your cash is limited so you must maximize the effect of each tower. There are two keys to achieve that.
Place towers so that the bots must follow long zig zag paths from one screen side to the other. Gun tower's effect is thus doubled in the beginning, rockets can reach four such paths each.
Slow towers are placed so that all fast units (air and land) are always slowed. While in the first waves armour is hardest to stop it is speed in later waves. Armoured units can accelerate a bit between slow towers, fast units must not. Slowtowers reach two paths in the zig zag pattern so they only have to be placed in every 2nd row. Their spacing depends on how far you upgraded them. It is essential that slow towers are in the paths of air units. You may have to find a good compromise here.
SAMS are usually best placed in the center of a map. First place them where they intercept air units on multiple paths (if applicable), then place the one that concentrate on single paths.
Teleports are placed in front of the exits so that they catch air units.
Gun towers are placed to channel land units into the teleports. Slow towers are placed along the teleports to slow units there (see below).
Breaking the waves
Start building the zig zag from gun towers and slow towers. In the beginning a units maximal progress depends mostly on its armour. Tanks are the first hurdle, they appear around wave 10. When the zig zag is long enough to keep them contained, save money for the next hurdle: first air unit appear from wave 15. If you can spare the money before wave 15 build rockets that will be converted to SAMS later but also cover the zig zag now. By wave 15 you must have sufficient reserves to place a teleport in from of whichever exit the air unit will take.
From then on you must always keep a reserve to place additional teleports (up to five in a row or so) or at least replenish them. The slow towers beside the teleports give you a bit more time to replenish teleports while a wave crashes into your teleport row. That way 3 teleports can zap 10 jets back to the start (if you are fast and the jets not to closely lined up).
The next steps are building a SAM, finishing the slow tower placement, upgrading the slow towers in the zig zag, swapping out strategic gun towers in the zig zag for rockets and upgrading those rockets (upgrade before swapping out more).
The end configuration is about the equivalent of three vertical lines of fully upgraded rockets and slow towers, rows of four or five teleports in front of each exit slow towers in the air unit paths and beside the teleports and 20 or so fully upgraded SAMS (placed to also extend the zig zag a bit).
The art is balancing everything. If no land units never run into you teleports you invested too much into land defense, same for air. Strive for perfect game, it pays well and it's your only chance to see the Titan (you always see it in higher levels when you're perfect at wave 70 ... or 80?).
Upgrading
Upgrade SAMS/rockets to the max first, then build/swap more.
Focus your rewards in Rockets/SAMS/Explosives. Also spend some on gun towers, slow towers, cash and health. Obviously yu have to unlock the teleport to use my strategy.
Earn achievements, it pays well.
Enjoy!