As others have basically said, the enemies start off too strong. If they started off easier it would give the player more time for their initial strategy, then as they got more difficult it would become more frantic as the player tried to keep up with the challenge. However, it is easy to get stuck with a full board and not be able to continue, so some method of clearing part of the board (maybe at the cost of your best tower as a trade-off) would keep the game going.
Otherwise, I really like the game. It's an interesting blend and the presentation is pretty good. I find it's more of a 2048/3s game with something extra, rather than a tower defense missing a mechanic though.