Congratulations on the Kongregate badge. Just a recommendation in case of a sequel: Either offset the entrance to a secret/hidden pathway or give it a very slightly different tile set so that it's not obvious something is off but you can still tell. Was not a problem for this game as all the secret paths were intended to be taken; but a good thing to keep in mind in case of a bigger sequel with a hidden ending/Easter egg.
Anti-frustration/keep gold relevant after all upgrades have been purchased idea: Once all upgrades have been purchased, have one final permanent upgrade that increases your health by 1 each time you buy it but the cost goes up after every purchase. Something like 20,000 up front and +5,000 to the cost after each purchase. Don't offer it for town health however, that's how you keep the game interesting (and allow an easy option for death to power levelers).
The amount of gold you need to ante-up to the next level are 200, 1000, and 2000 for levels 1, 2 and 3 respectively for those trying to get to the King of Greed as quickly as possible. If you have any less when you die, you won't have the option to ante-up to the next level.
Would be nice to have buttons like the Ultimate Smash for Hell Raiser and Meteor Smash instead of them being sensitive to your height. Too many times have I preformed a Meteor Smash because I was not below the invisible threshold.
Something of an exploit: if you deliberately crash land and shut your engines off (hold down S) you can make a lot of money just through duration by just sitting on the ground and enabling engines just before the timer finishes with
"END" for a brief moment to reset the timer repeatedly.
It would be nice to be able to switch between guns to conserve ammo. Like being able to pull out the shotgun when the boss shows up and switch to normal gun on grunts.
Also something to consider to increase the difficulty later in the level is have the guy start running faster.
For those who think the game is too dark (sight-wise), explosives light up the whole screen and there's no splash damage to yourself for using them point blank. Invest into explosive weapons and spam shooting to keep things lit up.
The trick to Tower of Glory is that EVERYTHING follows the same pattern. Time yourself and adjust every time you die. Do that and the only hard part will be the "jumps" to hit the switch.
Now that we can set rally points, the next step is to add advantage/disadvantage terrain so that we have a reason to place soldiers in places other than choke points.
This game isn't too hard if you take advantage of the design. First off you only need to play 1 mini-game to rack up multiplier so lose every game intentionally but 1. That'll help keep the jarring game switches from being too bad. Whatever game you decide to play, make sure it isn't random (you can see what will kill you right from the start). I recommend Avoid Green myself, it's one of the easiest non-random games and I got to a 10x multiplier and could still easily dodge the Greens. The speed and position of the Greens is determined randomly at the start but consistent for the rest of the game. With that in mind, keep losing until you get a game where all 3 Greens are moving at the same speed (preferably a slow speed) and are close together.
A limited version of the Antimatter Shield should really be a feature of the game with the upgrade option of a non-limited version like the upgrade currently is. Otherwise this game is way too luck-based on getting bullet patterns you can actually block.
Go for 4 Homings (weapon 8) first. It'll be useful throughout the whole game. Then get Energy Homers (weapon 12) and switch between the missiles while one or the other is reloading. From there it's your choice between Lightning (weapon 16, great crowd control), BFG (weapon 17, fast reload good while other weapons reload), Big Laser (weapon 20, selective crowd control), and Duck Death (weapon 21, incredible single-target damage). Ignore my boss advice, Militant8invader's is better.
Tips: Go for 4 Homings (weapon 8) first. It'll be useful throughout the whole game. Then get Energy Homers (weapon 12) and switch between the missiles while one or the other is reloading. From there it's your choice between Lightning (weapon 16, great crowd control), BFG (weapon 17, fast reload good while other weapons reload), Big Laser (weapon 20, selective crowd control), and Duck Death (weapon 21, incredible single-target damage). For last boss use Duck Death, Laser Lock (weapon 14) and 4 Homings (he moves too fast for Energy Homers) and stick to the edges.
This has already been said by other people but to sum up: 1. Stick to the sides and use strafe to fire towards the middle/edges. 2. Use bullet time whenever you need to dodge bullets even if just for a second. 3. Use berserk if you get trapped (it turns enemy bullets into money). 4. Aim for purple stars first, they move a lot and fire erratically.
Easy way to get the hard badge/gain money until you can buy good cards: After you've beaten everyone on level 1, make a deck that focuses on Goldmines and Peasants with some mana crystals so you can play Parting Gift. Make sure you'll have at least 6 Peasants for 120 total easy to kill units. At the start of a match, build up gold and crystals (go ahead and let the enemy hit your portal a few times if needed). Kill off 2 Peasants so you can play Parting Gift. Now just stream a steady flow of Peasants so the opponent will kill them off without the Peasants getting off attacks. 102 (you needed to lose 2 to play Parting Gift) Peasants later, you've won.
 "Labyrneath"){kind=icon}
 "Ante Hero"){kind=icon}
 "Ninja Slash"){kind=icon}
 "Berzerk Ball 2"){kind=icon}
 "Into Space 2"){kind=icon}
 "Infectonator 2"){kind=icon}
 "Nightmare Runner"){kind=icon}
 "Decision"){kind=icon}
 "Kill the Heroes"){kind=icon}
 "Awesome Tanks"){kind=icon}
 "Amberial"){kind=icon}
 "Kingdom Rush"){kind=icon}
 "Cursed Dungeon"){kind=icon}
 "Grid16"){kind=icon}
 "Shield Defense"){kind=icon}
 "Heavy Weapons"){kind=icon}
 "Death vs. Monstars"){kind=icon}
 "Cardian"){kind=icon}