CastleStorm, the physics-based seige sim which appeared on the PS Vita last year, will come crashing onto the App Store this spring.

Combining the object-flinging element of Angry Birds with the base-reinforcement mechanics of a tower defence game, CastleStorm encourages you to erect mighty battlements, while simultaneously shattering your enemy's structures to the ground.

Unlike the Vita version, this iOS port of Zen Studio's 3D smash-'em-up will be free-to-play. Despite this, the developer claims the game will still include four campaigns, four playable nations, and 150 different battle scenarios.

Mobile gamers will also get access to previously unseen spells. We doubt one of them will dispell the freemium model, though.

Expect to see CastleStorm on iOS devices before the end of May.