While an amusing game that I enjoyed very much, it IS rather repetitive and easy.
1) Upgrading is always better.
2) DPS/gold spent is most important as well as granularity of damage; sparrows and goldfinches are your friend.
Once you have 4+ full upgraded sparrows, stock up on fully upgraded goldfinches and thereafter fully upgraded turkeys.
The blinking teleporters gave me a headache, and I'd imagine some people react far worse. Please provide a seizure warning for the consideration of your players! Otherwise a great story, if somewhat short and sometimes frustrating. I'm looking forward to seeing more!