This is a very decent little game that looks like it's suited to mobile platforms, but has far more depth than that.
Castlestorm is a mix of real-time tower defense and sidescrolling hack'n'slash, but the former is much more prominent when you actually play the game. The gist of it is that you need to prevent enemies from walloping down your castle gates and taking your flag back to their own castle - everything is on a 2D plane, a straight left/right path. You can do this in multiple, often very creative ways - but the three staples are using your castle ballista to shoot enemies manually, sending out your own soldiers to die valiantly in the line of duty, and controlling your own powerful general out on the battlefield. While the latter is fun, the castle projectile system is what you'll be using the majority of the time.
You'll be using your array of projectiles to deliver more headshots than Liebovitz, but it isn't just limited to bog-standard ballistas. Yes, you can fire a potion to turn that enemy griffin into a harmless turkey, and yes you can shower that viking brute with a barrage of kitchen utensils. The options are surprisingly varied and most of them have their uses, with different factions having differing projectiles available. You unlock these, the soldier classes and other spells/buffs by accumulating gold throughout the campaign and skirmishes, which are then used to upgrade your wares. These also include castle rooms - ballistas can fire at the opposing castle as well as enemy units, and if a room in the castle is destroyed, then whatever benefit it gave to your forces is immediately gone. This can include your army, of course...
The campaign is a good length, and the missions are reasonably varied although it might get slightly wearing towards the end. I would say that the DLC is worth it if you enjoy the game, as it's pretty cheap but opens up two new campaigns and factions to play around with. The story is pretty standard questing fare, although it plays around with tropes in good humour. Multiplayer is almost completely dead, so count on a single player experience only.
On the whole, Castlestorm is a good game that defies expectations. It looks like a kiddy iOS game, but actually gets quite difficult at points and utilises its own game mechanics well. I'd recommend it to most people who fancy a somewhat casual 2D tower defense. Also, Freia is awesome.
'They've smashed ah rhume in yer cah-sull!'