Adds a challenge of launching different attack rockets to avoid destruction of your base.
Enemy ships scan the planet surface for strategic attack points and coordinate nuke drops.
Specifically they target rocket silos to cripple your combat capabilities, and also dense areas of player entities to maximize damage to the factory.

Once player has set up automatic defenses that consistently manage to quickly defeat all ships, the difficulty is low. However failure to set up defenses in time could result in destruction of the whole factory permanently.
Difficulty can be tweaked in settings.

Attack wave happens after a random amount of time. Once multiple nukes are dropped, there can be a momentary slowdown, but after that the game runs smoothly.

Background:
You have violated the Prime Directive for financial gain and will now face the consequences. Fast evolution draws attention of extraterrestial ships, resulting in them nukeing the factory that interferes with local wildlife. In post-launch combat you can use ballistic missles, intercept-capable lasers or even launch the rocket with a nuclear payload at federation ships.

Base code and graphics for ballistic missiles by Dave-1993_