At the last count, there are as many tower defence games on the App Store as there are cells in the human body, and this makes it difficult for new titles to stand out.

Human Defense tries to do it by taking you deep into the human body, where there you must guard vital organs from the attacking pathogens.

It starts off as a pretty mediocre tower defence game, but as you play it develops into something richer and more challenging.

Playing in vein

The different types of pathogens all have their own strengths and weaknesses. While you're briefly told in the tutorial what these differences are, the pathogens all look very similar and so it's hard to differentiate between them.

If they get past your defences, they then come back in a stronger, mutated form, ready to face your towers again. After a number of mutations, they can eventually damage the organ you're protecting.

It's a fun twist on the tower defence formula, but the mutated forms of the viruses are vastly overpowered in comparison to your towers, which are very weak and slow. Tower upgrades aren't offered until later in the game, but even then the boost to their power is negligible.

Using nutrients from the bloodstream, you can build up to four different types of tower. Unlike in traditional tower defence titles, you can only place a few of them in each level, and only in prescribed spots.

While infuriating at first, especially for regular tower defence players, this constraint brings a welcome focus to the game as you weigh up the relative merits of the potential tower locations.

It'll knock the air right out of you

But the game doesn't really shine until it introduces some richer strategy elements.

In later levels, you have supply nutrients to the organs themselves to maintain balance in the body. Supplying the right amount of nutrients even allows you to recover lost health.

But getting nutrients to the organs means bypassing your towers, which can leave them unfinished or un-upgraded. It's a fine balance, and one that's very satisfying to achieve.

If your towers aren't up to the job, you can use coins earned by completing previous levels to buy power-ups that can boost your damage output or reduce enemy speed. These boosts are expensive, and not worth purchasing unless you're desperate to three-star each level.

Human Defense isn't a great tower defence game. But eventually it introduces extra strategic elements that help to balance the gameplay. As you may have found with your own body, you'll have to put in some time and effort to get the best out of it.