This would be a really good game if not for several small annoyances and, most importantly, the fact it's a major grindfest. It's possible to beat without spending money, but it takes forever, playing the same levels over & over & over for money and XP to be able to get strong enough to proceed. There are also issues with difficulty balance/spikes, and most of the game really isn't much of a challenge, even on nightmare mode. All you need is maxed out arrow, air, and defender towers for 99% of the game, and possibly mortar and stomp towers for one or two levels. There's more difficulty in dealing with some of the game's idiosyncrasies than in actually beating the levels. If they'd focused more on making the game balanced and properly challenging (instead of hard just because you haven't spent enough money or enough time grinding) instead of just trying to get people to spend money, it could have been 4 or 5 stars. Instead, it's only ok and not worth bothering unless you don't mind spending the time and/or money.