As I play the game, I notice that eventually tougher enemies show up. From what I've read, it sounds like this is based on elapsed time rather than "research checkpoints", but I haven't seen any specifics. What kind of time frames are we talking about for various enemy types to appear? Are there any fixed enemy events that happen after a certain number of days? Does difficulty level affect this?
I want to have an idea of when I can expect so that I can better plan my strategy.