After some scientific research, the whole game is reduced into a spread sheet. There are not enough randomness to sustain the game. Only the spawn creeps have some randomness in the way they spawn into several smaller creeps. Also the creeps are dumb. Once you figure out a good way to kill the creeps, they are all dead sooner or later. The mazes don't make a lot of difference when at the end it's all about fire power.
Most weapons are smoke screens on the hard mode for example. You have little choice on a few of the useful weapons.
After I killed all the creeps with ease, I researched how the highest scores are achieved. I think they are smoke screens too. The scores with videos to back up aren't that high. I think many high scores in the scoreboard are categorized by mistake or deliberately. I looked at the associated mazes. Though mazes don't tell you the full story because it's a snapshot of the final moment, but some mazes contain too much money in it for the hard mode. Maybe the money is possible at the very end, but then it's unrelated to how to pass the most difficult stages.
After so long, I should think the highest score gurus would have come out to explain how they invented the winning strategy and earn the bragging right. Perhaps they want to keep it a secret because the game is so simple. Once they tell you everybody know how to beat it with ease, unlike every other console games. Perhaps the scoreboard is really one big smoke screen to keep people playing.
The challenge mode and fun mode are giveaways. The game itself doesn't have that much variation. By using totally different rules, you are playing different games when the original game is done and dead.
Killing all creeps gives you about 7,000 points. If you send the creeps earlier you earn more points. These extra points are about linear, more or less the same at every level. So to get some of the high scores, almost you have to send all creeps at once, which isn't possible. I estimated the max scores possible is about 10,000. I don't know how some 14,000 can be achieved in the hard mode.
So this is my final take on the game, unless some guru come out and explain how they earn high scores. And it have to be interesting, that is, related to real war strategy.
Swarm tower: it's much more efficient for air defense than squirt towers. It had to be at the dead center of the desktop. When you have two, put the 2nd one next to the dead center. The upgrade sequence is simple. When you are at level 7xN, your tower should be SW N. But you don't need any swarm for the first flying creeps when N=1. This assumes that you have a strong enough squirt tower to help with air defense, not busy with other creeps.
For level 7x7, you need two equivalent sw6 towers, such as one sw6 touching two bo5 (50% boost) towers. For level 7x8, flying bosses, you need the equivalent of four sw6 towers, such as two sw6 touching four bo5 towers. Also for the flying bosses you need two frost towers, fr6, to cover both boss.
There could be variations that I didn't exhaust. If you only slow down only one boss, each boss in theory absorb twice the fire power. I did tried with one fr6 to slow down one boss. It worked pretty good but not very consistent. If you need two fr6 to slow down one boss, it doesn't save you $250. You can have five bo5 (maximum boost towers) touching one sw6. It's not too good but it works better with an extra sw5 touching four bo5. I didn't exhaust all these, but the money saved doesn't seem to be that great, so you don't gain a lot of fire power for the other creeps.
You need only one bash tower, the priority of upgrade. It kills all creeps all at once passing through, the strategy to have high scores by sending the next wave of creeps early. But you cannot have bash alone because they cannot kill any boss for their short range. The best "maze" for bash is the cross but degenerates into one square block, and you try to surround the bash so creeps have to run along it on all four sides. See DNA defense in earlier post. But this conflicts with boost towers, which has to touch the bash.
The compromise for the bash maze is for the tower to touch two (or even three) boost towers. There are still three sides for the creeps to run along. This still look somewhat like the DNA defense.
You need only one squirt tower. You need a long range weapon to kill the bosses, and certainly the spawns. It's sufficient if this tower is boosted by two (or three) boost towers. The best maze is a cross three square in width initially, and five square in width at the end. You force the creeps to run around your cross. Your cross have to be small to survive the initial attacks with little money, and enlarging your cross is easy and cheap to do, but very effective. You should integrate one corner or one side of the cross with the bash maze.
You upgrade your squirt along with the bash. You need a spread sheet to be optimum. But simply put, you upgrade when you have money, bash first, until you bay $290 for the squirt before $260 for the bash. And it doesn't matter that much if you are good.
So you have one bash, one squirt, and one swarm. When they are all upgraded to the full, you try to touch them as much as possible with 4 or even more boost towers. When you get to the flying bosses, you need in addition one sw6 touching 4 bo5, and two fr6 covering each flight path.
You also need the so called juggling to send the creeps back and forth at some point. With my sufficient fire power, you can keep juggling to a minimum. Juggling means you don't have good design and enough fire power. The creeps will accumulate and divert fire power if you juggle too much.
For the last two bosses, you must send them out together, so no remaining creeps are to be released. All maze blocks part of the creep's entrance so they are forced into the heart of the maze. When no more creeps are released, you can just unblock the entrance, and block the exit of the maze, so the creeps have to go all the way back to where they started. For a good design they will be killed before they reach the entrance. There are no extra maze needed for juggling.
The only other case you need juggling is for the spawns, especially for the bosses at level 48. But since the next creeps are flying with no more creeps coming out for a long time, you can do the same as the final bosses. But if the spawns aren't killed soon enough, the squirts will be diverting fire power from the flying creeps. Anyway, a little juggling can save you a lot of money for other things. In practice, you need to add a little extra maze such as the DNA pattern to slow down the creeps at the entrance and exit, to give you enough response time to sell and buy blocks.
One possible variation, if boosting the frost towers further slow down the creeps, it would be very useful. But I doubt it, the game isn't that complicated, such as that the boost towers don't boost each other. And if the slow down can be boosted, the game would be too easy.