I suggest making it clear at the start of each level which squares can be used to construct towers. Maybe a colour-coded grid system could help, where red squares represent squares which need to be unlocked via cemeteries/training camps etc, and green squares represent those that have been unlocked.
Please buff Shield Wall because it costs too much power and lasts for too short a duration! Also, it shouldn't be cancelled when you merely pass a turn.
This game is really imbalanced. To top it up, it's a grindfest. Doing the same thing over and over again is really repetitive and boring. I'm not coming back to this game until the gameplay is improved.
I finally managed to 3 star everything (on normal) after many tedious retries. The best (and cheapest) units to use are lightning, sprinkler and ivy. The early - midgame is probably the hardest, as you do not have access to the better towers yet.
You may want to allow users to drag multiple units instead of manually clicking individual units which can be tedious at times. Also, maybe improve on the AI a little, such as making ships automatically hide behind barriers etc.
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