Enemies

Gliders
Shoot them. They will target anything that shoots at them first. They can be destroyed by bombers from your airport. One of them is more than match for one soldier in a turret, but a single laser is enough to destroy one of them.

Scouting gliders
Variant of one mentioned above, sometimes appearing in the first wave. They are easily recognized by their yellowish colour, and they are harmless, although they still glide over your modules in same manner as their armed counterparts do.

Helicopters
Fear them. Helicopters have a longer range than lasers but not turrets. They are also slower than Gliders so they arrive at your base late after you think the wave is done. Zooming out during a wave can help you spot an incoming helicopter for early detection and to prepare for it by building necessary turrets near where it will attack. They are also significantly tougher than gliders, being able to withstand multiple lasers if a turret lures them in range of the automated defences

"Sad Robots*"
Run and hide. These walkers are very durable, moreover: once destroyed they deploy a squadron of gliders. Fun. Some levels require you to bomb them and destroy the gliders they release at the same time. To do this, designate 2 or more bombs to be dropped in quick succession on the Sad Robot or else a stray glider may escape and most likely destroy your base. If possible try to drop 4 bombs to maximize chances of success.

* I initially used the name "Walking Death-Things" but yiotro uses the much more adorable "Sad Robots"

Drones
These ones are fun. Airplanes forever. Drones fly over your entire base and plant bombs. Use carrier units to move the bombs to an airport or junkyard. Sometimes multiple junkyards with each set to accept and burn a low quantity of bombs is the fastest way to clean them from your base before the next enemy attack and destroy a module with a bomb still in it.

Mosquitoes
They swarm out of nowhere and slowly kill all units in the base. The only defense is building a biobait tower, which after it detects a swarm will attract and destroy all mosquitoes everywhere on the map.

Enemy Bases
They're like cancer, kill them early. A base can grow from any type of enemy module. Typically they grow in the direction of your base. Even one module of enemy base is enough to make it expand, therefore requiring complete destruction or regular bombings of freshly built modules are required to keep them away of your base.

A strategy that can be employed is destruction of the majority of the base but leaving 1 spawner left standing. This is useful where the tasks to complete the level include both surviving waves and destroying the enemy base. If the base is destroyed before all of the waves are completed, new enemies from a new wave will spawn at the edge of the map in random locations, whereas if 1 spawner of the enemy base is left alive, all enemies that spawn from a wave will come from that spawner. This makes defending easier because defensive modules can be placed in the direction of this last spawner.

Platform
Connects other parts of the base.

Spawner
Produces enemy gliders and drones.

Minigun
Enemy miniguns have a range longer than lasers, but shorter than turrets. They do a high amount of damage.

Laser
It has a long range and it shoots anything that comes near it, if the enemy base has a laser the only method of killing them is by making peace with them

Enemy Behavior
"Enemy" pretty much describes it.

Some maps have gliders already spawned where they just fly around in a small area only leaving the area if attacked.