There's a lot that could potentially go wrong with CastleStorm - Free to Siege.

Firstly, it was originally designed for PCs, consoles, and handhelds, and not from the ground up with touchscreens in mind.

Secondly, it started life as a paid game before undergoing the often messy transformation to free-to-play.

Finally, it also contains not one main type of gameplay, but three.

Yet, miraculously, not only does CastleStorm - Free to Siege get away with all of the above, but it manages to be one of the better genre mash-ups I've played in recent months.

Storming the castle

The plot revolves around a long and bitter war between two neighbouring lands, which ceases one day thanks to a pair of special protective gems falling from the sky. One gem goes to each side, defending the kingdom each is housed within.

Then one of them gets nicked, and it's up to you to restore harmony.

The story in CastleStorm - Free to Siege is fine, but the humour is much better. Before most levels there's a short in-engine animation in which the various characters bemoan one another's cowardice, foolishness, stubbornness, and so on.

There's a scene with a sheep lodged in a trebuchet, a scene with a queue for a single latrine, and so on. It's all quite cheeky, and good for a few laughs.

The gameplay is split three ways: an Angry Birds-esque fling-'em-up, a simple hand-to-hand combat mode, and a tower defence-esque game of tug of war in which you produce units to attack the opposing army.

The latter two of these are straightforward but enjoyable enough. Some levels feature all melee combat, in which you control a hero who runs about the 2D stages swinging their weapon and defending themselves with a shield. Other levels see you creating troops to send out and fight for your side, the skill being to predict what the enemy will throw your way next.

It's the ballista that you'll have most fun with. You pick the ammo you'd like to fire, then tap where you want to shoot, and hopefully you'll hit a marauding baddie.

Combo

However, it's when all three are combined in one level that the game is at its best. Firing upon the enemy base with your artillery, then sending out a bunch of units, then summoning the hero to fight alongside them, then switching back to the ballista to continue providing support, is fast, frenetic, and fun.

If there's an issue here, then it's that on mobile the buttons can be a little fiddly to tap - especially in the heat of battle. This issue affects the aiming of weapons too, though you can always zoom-in for a more accurate shot.

When not battling, you're levelling-up and improving your castle, giving you access to more powerful troops and diversifying your arsenal.

Pretty soon you're taking pot shots at pesky flying enemies, healing your troops with the Priest, and demolishing enemy walls by lobbing large rocks at them. There's plenty more content on top of that too, and each mission rates your performance out of three based on meeting special requirements - such as losing less than X number of soldiers.

Even though, on paper, CastleStorm - Free to Siege is a hodgepodge of ideas, it's shot through with quality and class, making for a novel but satisfying take on the tower defence (and physics-puzzler, and scrolling brawler) genre.