There's probably a few of you out there who will flinch involuntarily at the mention of Desktop Tower Defence. It will, if played properly, sap hours and hours from your otherwise ruined life, as you click incessantly and mentally debate such questions as "should that tower have been about three millimetres to the left?"
For those who are unaware, "Tower Defence" is a type of game that requires you to prevent some kind of creature from crossing the screen by placing various types of defensive towers along the way. Each tower may have a certain type of attack suited to particular creatures, and probably some kind of upgrade path, and generally the objective is to either destroy the enemy, or prevent them from destroying you. The concept is thought to have really been popularised with the Warcraft III map "Wintermaul" back in 2002.
In PixelJunk Monsters, you control a kind of... well, I don't know what the hell he is. Possibly a turtle crossed with some kind of American Indian chief. Whatever, it doesn't matter, the objective here is to stop the monsters from making their way across the map and killing the 20 villagers you're in charge of.
You have three towers you can build on any of the trees in the map - an arrow tower with a weak attack that hits any land or air creature, a cannon with a large attack and damage radius that can only affect ground creatures, and an anti-aircraft tower with a moderate attack that - you guessed it - will only hit flying creatures.
Each tower requires a certain amount of gold coin to build - arrows are the cheapest at 100 coins, cannons are next at 120 coins, and anti-aircraft are 150 coins. If you manage to kill a monster, they will always drop coins (with the exception of one map) and on occasion, gemstones.
Gemstones really are the most important items in the game. They can be used to purchase new types of towers and special features, or they can be used to improve the damage, range, or rate of fire of your existing towers.
When you initially build a tower, it has a small blue flag next to it, indicating that it is the most basic form of that tower. You can improve it by two methods - either sit your turtle-Indian creature on top of it and watch him do some kind of crazy dance which gradually levels the tower up, or you can simply select the tower and spend gemstones to instantly upgrade it.
When the tower is upgraded to the next level, the old flag drops and a new one pops up in the order blue, yellow, red, purple then black. When spending gemstones, you only need one each to go from blue to yellow, and yellow to red, however you will need two from red to purple, and three from purple to black, meaning a black tower of any type represents a significant investment. Happily however, you are rewarded well for your investment, with black towers almost always doing a prodigious amount of damage at a good range.
Each level of upgrade will only affect one attribute of a tower, so for example with arrows, you can upgrade from blue all the way to purple and it will simply increase the range at each level. You need a black arrow tower to do any more damage than a blue one. Similarly, a black cannon tower has no more range than a blue one, but it does a huge amount of damage in comparison.
Assuming you're all suitably confused, we'll move on to the creatures. There are really only six types, plus the end boss. There are fast movers (bats and spiders) and slow movers (apes and some kind of enormous bee creature), and two moderately paced monsters (some kind of mosquito, and what looks like a round blob with legs).
You'll need to take notice of what the next wave of creature is, because if you don't have any arrows or air turrets, a single flying creature will kill all your villagers whilst your muted cannons look on. To complicate matters further, some land creatures will spawn with either a resistance to flame, or a resistance to physical damage, so you will need to investigate alternate towers to purchase on most maps;
* Arrows: land creatures only. Moderate damage, good for spiders and bats.
* Cannons: land creatures only. High damage, good for blobs and end bosses.
* Anti-aircraft: air creatures only. Moderate damage, good for mosquitoes.
* Fire tower: land creatures only. Huge amount of damage, essential for physical resist creatures.
* Laser tower: air creatures only. Huge amounts of damage, essential for enormous bee creatures.
* Tesla tower: land creatures only. Moderate damage, large radius, not really used.
* Ice tower: land creatures only, slows down creatures. Not really used.
* Lightning tower: All creatures, moderate damage, slow rate of fire. Not really used.
* Hive: land creatures only. Not really used.
* Mortars: land creatures only. Huge amount of damage, only really needed in one map.