In fairness, the "gems" system isn't unique to this game. I've seen it in quite a few others, especially in the Tower Defense genre. Where it gets particularly

In fairness, the "gems" system isn't unique to this game. I've seen it in quite a few others, especially in the Tower Defense genre. Where it gets particularly annoying is in games with difficulty curves I can only assume were designed to require more resources than the player is allotted naturally, thus forcing him to buy gems and trade them for upgrades. It's not literal payment-gating of advancement, but it is de facto payment-gating of advancement.