I liked there was a separate scaling cost for upgrades and new towers, encouraging use of both. It wasn't quite clear what the exact difference between the cannons and ballista were apart from fire rate. The projectile "over-kill" as it seemed was confusing as it wasn't explained to me leaving be confused as to whether it was a bug. Despite those confusions it is quite simple to understand and I even did a retry to see if a different strategy worked better. - Devinant