Kingdom Rush is the first entry in the series of strategy tower defense games, but the first game still holds up well and is widely regarded as an excellent title. Kingdom Rush is downloadable for free and features in-app purchases.
Con In-app purchases for virtual currency
Kingdom Rush has some in-app purchases to unlock new playable heroes - which is understandable considering the game is free to download. However, there are also microtransactions available in exchange for in-game currency which can be used to buy temporary power-ups.