Read the in-game help for an overwiev of the in-game controls. If you are familiar with TD games, you might just fire up a game, it's not more complicated than your usual tower defence.
Star Temple TD is a survival style game. That means, it has no end, no last wave. Enemy ships will come at you endlessly, with ever increasing strength, eventually overruning you. Point of the game is to hold out for as long as possible.
Ship defenses: Ships have two layers of defenses, shields and the hull. Shields keep regenerating over time, while the hull, if once damaged, stays damaged. A ship dies if the hull is destroyed, regardless of the status of the shields - certain events damage the hull directly, bypassing shields. The strength of the shields varies across different classes of ships.
Projectiles: these are the cheapest weapons. They have fast tracking, decent damage, and their kinetic impacts can push ships, hindering their advance. The largest tier projectile tower's special trait is, that it can fire omnidirectionally, thus it does not depend on tracking the target at all.
Beams: these lazors deal lots and lots of damage, but have quite slow tracking, so good placement is important. As they are focused energy weapons, shields are badly strained when being hit by them, suffering extra damage.
Missiles: unlike the other weapon systems, missile towers do not rely on tracking to be able to hit their targets. They also have the longest range, and missile explosions damage all ships in their radius. Their shockwaves also push ships, but are less predictable than projectile weapons. As the explosions produce a large number of scattered kinetic impacts, shields have an easier time absorbing them.
Shield disruptor: technically, it is a missile class weapon, but unlike its small and medium counterparts, this one is specifically designed to neutralize shields on a large scale. Huge area effect. As a drawback, this massive energy torpedo does only negligible amount of damage against ship hulls.
Kinetic hits: projectile and missile weapons have kinetic hits. This means, their impacts can push ships depending on the direction of the impact. Projectile weapons with good placement can be effectively used to slow down ship movement. Missile explosions push ships outward relative to the center of the explosion, so they are less predictable. Keep in mind though, that over time the ship's navigators will compensate for the impacts, thus it is impossible to pin a ship into a corner for ever.
Environment damage: Outside of direct weapon fire, there are a couple of things that can damage ships. This kind of damage always directly damages the hull, bypassing shields altogether. One of them is collision with the walls. Damage caused by it gradually diminishes in proportion to the ever increasing strength of the ships, so it is more of an aid in the first waves, but becomes negligible later. The other kind of environment damage is ship explosions. When a ship is destroyed, the final explosion damages other ships in it's vicinity, similar to a missile explosion. The amount of damage dealt is relative to the original strength of the exploding ship. So look out for those heavies.
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