Quite a fun incremental game! Fun game-loop of shooting down missiles, collecting resources, and upgrading. Great music choice as well.
Some ideas include:
- Some enemy types: maybe a bomber plane that drops multiple missiles unless its shot down, maybe a 'cluster' missile that splits into multiple missiles shortly before the ground doing lots of damage, etc.
- I think it would feel a bit better if some of the 'waiting' time was cut down. I would never buy the "increased safe period between waves" as it meant less research and missile drop income. That upgrade would feel much better if it was just "instant 10s of progress to all buildings between waves". Possibly also a skip button for ashfalls other than the first.
- Have an upgrade for each building type that sets/increases the maximum number of that building active would help smooth out the progression curve.