End game combat ideas about missiles and alien fortresses

[Zentay]

Introduce the following to the game:

- Ballistic missiles. These are costly to produce.
- Enemy fortresses. These are bases that are especially well protected and difficult to destroy without ballistic missiles. They are protected by artillery worms that will target the player's structures within a large radius. This means you cannot build you own turrets near these bases to easily destroy them.

Change the drop rate of alien artifacts so that they are abundant in fortresses but scarce elsewhere.

Change the alien behaviour so that destroying enemy buildings that are part of fortress will trigger a large counterattack by the remaing guards. Conventional weapons are still needed to survive these counterattacks.

Alternatives: instead of missiles, there could be bomber drones that fulfill the same role. See https://en.wikipedia.org/wiki/Unmanned_ ... al_vehicle for ideas.