Card Wars - Adventure Time is a digital card-scrapping game starring the cast of the universe's favourite cartoon. And it's actually quite good, albeit with one very big problem.

It's part-deck builder, part-lane defender, part-reaction tester. And it's presented in a thick-lined cartoon aesthetic that looks like it's been lifted wholesale from an episode of the show.

In the campaign mode, you battle through the cast, learning the ropes of the game. You lay down a variety of landscapes to make your side of the playing board, then add monsters and buildings.

These are your lanes, and they're reflected on the other side of the board by your opponent's. Now, you hack away at each other.

An empty lane leaves you vulnerable to attack, and you only have a few hit points. Lose them all, and you lose the game.

So far, so good.

There's a decent depth to proceedings - you earn more cards to strengthen your deck as you beat your opponents.

The problem is there's some pretty ugly free-to-play stuff 'hovering' around the main game. There's an energy system, and a gem-based currency available in bundles costing up to £39.99 / $59.99.

And this is in an app that costs £2.49 / $3.99 upfront. It all feels a little uncomfortable, and spoils what would otherwise be an engaging little card game.

Card Wars - Adventure Time is available from the App Store right now. But I'd have a good hard think about whether you want to spend the £2.49 / $3.99 on it.