Almost all levels can be won by using startig energy to build 3/4 light carriers and 1/4 miners and nothing else. A few need a bit of a nudge from that for initial defense, but mostly the first carrier wave will be sufficient (if carrier upgrades were purchased, particularly) Miners will keep income high enough to permie additional light carriers to be built.
I just won on "Hell". The basic strategy is to switch to lasers as soon as possible and use them exclusively for damage dealing. The only hard part is managing to switch as early as possible without wasting resources on other towers. In a way, the most difficult part of the strategy was realizing none of the other towers are useful. This isn't really a criticism, because I did enjoy the game and think the difficulty was very well calibrated, but I'd prefer it if the other towers were more generally useful. Maybe they are, and I just was playing badly, but on "Hell", it didn't seem anything else was possible.
The game is really well balanced. I can beat it on harder about half the time. Impossible remains just (barely) out of my reach. That seems about right. I used to think "harder" was pretty close to impossible (or luck based) but it really is possible to improve at the game. A general tip is to never be in a "consolidation" phase - always be on the move to increase your population, farms, etc. A school is close to critical. I'd say the early game is a little too position dependent on harder difficulties (although fun to have a hopeless setup sometimes). An easy 5/5 from me. Whining comments about the higher difficulties are a little annoying: Yes, it's a strategy game that requires some strategy (shocking!).
It's easy to win on the hardest mode if you follow this trick: Just send all cavalry down only one row. Only invest in cavalry and cavalry upgrades (and command point and income upgrade). If you send the cavalry down only one row, they'll mostly only get in one hit before passing out of the screen - that one hit has a bonus and will usually instakill (even against short-spear, etc). They won't always get out of the screen before the row is clear (and so can then be killed), but if you always use just the one row, you'll do well enough on average to never lose a battle (don't worry about the soldiers going down your other rows, I never lost due to that, although a few battles got a little close at the beginning before the row gets cleared for rapid transit).
All except the last one done on "completely crazy". I'll do the last now, though it'll be tiring. The trick for the motherships is to build distracting garbage as it shoots. These are the only level thels are useful for (with lots of batteries and energy) and a few missiles. For the transport modes, the trick is to not upgrade the initial builds and then sell them back for the original purchase price. Upgrade only the missiles for the motherships built very near the location where the warp in. The saucers can be taken out with tonnes of plain lasers (not upgraded) which can then be sold back for no cost. Sorry if all this is obvious.
This is the only game I've played on kongregate where I'm pretty confident that the most negative criticism is ill conceived. Playing through the levels was fun and well-balanced. An elegant game.
 "Obliterate Everything 2"){kind=icon}
 "Zombie Tower Defense: Reborn"){kind=icon}
 "Rebuild"){kind=icon}
 "Alexander: Dawn of an Empire"){kind=icon}
 "The Space Game: Missions"){kind=icon}
 "Effing Hail"){kind=icon}