I think the instructions would be more helpful if they included the price of each action and mentioned that the towers have a square range and target the closest enemy. None of that is fun to learn in a game that's this slow-paced.
Was the screenshot of the second map taken from the same build that was published? I'm not sure how you could last long enough for a third tower with that placement. The arrangement I managed to win with had the upgraded tower in a different spot and used a different part of the map after spamming the corner.