In the natural world, many pairs of living creatures are mortal enemies. Lions and zebra. Penguins and orcas. And, going by the tower defence game under examination today, there's no love lost between trolls and Vikings either.

Trolls vs Vikings by Megapop has a lot in common with a certain other game that has 'vs' in its name. The familiarity is both good and bad news.

The good news is that it's easy to get into Trolls vs Vikings with minimal guidance. The bad news is, it's impossible to avoid noticing how well-balanced the original Plants vs Zombies is next to Trolls vs Vikings.

On reflection, PopCap's casual masterpiece was probably an unwise game with which to invite comparison.

Don't feed the trolls

But to give credit to Trolls vs Vikings, the fantasy setting is a compelling substitute for undead beings and the salads they hate. Long story short, Vikings have invaded a peaceful troll village looking for gold and other goodies, and the trolls aren't eager to give it up.

The trolls aren't fighting the battle alone. They have numerous warriors at their disposal, many of whom boast character designs that are creative or just plain adorable.

Of particular note is a punk dragon that breathes fire in a straight line. He's the son of the World Serpent, and he's out to prove that both he and his rad haircut can hold their own in the big bad world.

Pay the toll

But although the creature designs for Trolls vs Vikings obviously have a lot of care behind them, the gameplay isn't quite as imaginative. The game thoroughly re-treads territory that's been previously pounded down by Plants vs Zombies and its millions of clones.

There are a couple of clever ideas, like tough brawler trolls that can be moved anywhere on the battlefield. But for the most part you're looking at the same old pea-shooters, 'wall'-style warriors, and miners that supply the energy used to buy your defences.

What's really unfortunate is that a lot of stuff is locked up behind paywalls. These include elite trolls, as well as expendable magic spells that drop rocks on the Vikings and erect walls of fire.

It's a trifle annoying when you're trying to choose your lineup for an incoming attack only to be told that you need to buy certain trolls.

Familiar fairy tale

Alas, as you might expect, the difficulty is skewed ever-so-slightly to prod you into making some in-app purchases.

For example, in Plants vs Zombies, your lawnmowers are your last line of defence: if a zombie gets too close to home, the lawnmower fires up and makes a single journey that's fatal to oncoming traffic.

Trolls vs Vikings lacks that failsafe. If a single Viking makes it through your warriors and into the village, you lose.

This isn't a terrible Plants vs Zombies clone by any means. Its fantasy setting is interesting, and its character designs are charming. But, as we found with the free-to-play Plants vs Zombies 2, there's no question that the formula works better as a paid experience.