Love the retro feel to this game! I could not figure out how to upgrade the towers though. The range on most of the towers are good, but I feel like the hospital tower's range was not perfectly clear. It was the only tower I did not use much because of this, and how high its cost was.
Mechanics were easy enough to understand, but I did wonder why I could only place a gunner as my first tower instead of being able to choose from all the towers.
The difficulty is good where its at currently, but I think either the costs should be lowered a tad or slightly more reward from the monsters. Especially with the bombers that destroy groups of towers, it can quickly eat through costs.
I ended up mostly choosing gunners and drones placing them 3-4 tiles from the core. Though even on my most successful run I only made it to level 11 (made it there 4 times out of a dozen or so attempts). I tried multiple styles including lining the core with traps, but I always felt like the costs were a bit unbalanced for me to be successful.
Great game otherwise! My main suggestion would be to balance the costs a bit and it would be perfect! No bugs from what I could tell though!