Can you stop the hordes?
You have 6 different towers.
The machine gun towers lay down a barrage of gunfire.
The RPG towers launch powerful grenades, but, can only hit walking enemies.
The "slow" towers slow down enemies for a short time - allowing other towers more time to hit them continuously.
The laser towers fire a beam of light that does plently of damage to powerful enemies and kills smaller enemies outright.
The Ion towers fire aerial and ground attack flames.
The Sniper towers fire long range beams that hit enemies on and off screen.
Each tower can be upgraded 3 times in order to get the fastest firing rates and the highest damage percentage - and at any time, a tower may be sold so you can build another tower elswhere in a hurry.
There are also repair bots. As the enemies swarm you, the structures you attempt to protect get damaged. The repair bots can repair structures, but their rpair work is very slow and it is likely your structure will be destroyed while they are attempting to repair it. However, the bots can also be tasked with resource collection which gives you money at a slightly faster rate.
You must learn to manage your money to build the appropriate tower in the appropriate place in order to kill as many aliens as possible before they reach points on the map where defenses are light and manage to destroy all your structures.
The aliens come in a few different types.
There are the crawling arachnids which hit in massive numbers and can be stopped by ground attack towers. There are mutants which are immune to "slow" towers and actually move faster when hit by them. There are flying monsters which move quickly and can only be hurt by aerial attack towers and then there are huge warrior monsters which carry a huge battering ram and can make short work of your structures.
The warriors walk slow but are heavily armored. As they only come on the 30th wave, your defense must be planned around setting up as many "slow" towers and high damage weapons as you can before the each consecutive 30th wave.
The problem with the game is the speed at which the difficulty ramps up.
Even once you have plenty of grenade launchers, laster towers, and machinegun turrets, at a certain point, the aliens move so ridiculously fast that there is absolutely no defense against them. No matter what you've got - the waves keep coming and you lose. Considering your repair bots work slowly and money comes in just as slowly while the enemies pour in, you are likely to fail.
The game is fun, but on its medium difficulty, its extremely challenging. You can choose to play the game's 4 "assault" stages consisting of around 90 waves, or, you can choose to play Endurance mode on any unlocked maps you've already beaten to see how long you can hold off the swarms.
Since the game is free, and its fun, its hard not to recommend, but, I'd like to see some improvements made.
#1 the enemies should not be so well armored, or so speedy unless the game is changed to give more money faster. One of those aspects has to change.
#2 It is easy to accidentaly sell a structure since you are touching the screen and your finger may get in the way if you have big fingers. Maybe it would be better to move the sell button to the corner of the screen?
OVERALL, its a fun game, and I recommend you play it if you like the genre. Even if you don't like the genre, its still not a bad game.