Lock's Quest follows Lock, a fledgling "Archineer" (architect/engineer, of course!) who is thrown into his kingdom's war against Lord Agony and his army of mechanical "Clockwork" soldiers. The Archineers' job is to build structures and defenses in order to repel the marching Clockwork and protect the "Source" that is both the energy and the material for all the structures. Luckily, the Clockwork enemies can only live for a few minutes at a time, so Lock only has to maintain a stronghold until they collapse.
Lock has a variety of walls and gates, and an increasing number of turrets and traps to accomplish this. Each locale (on the most beautiful world map I've ever seen) is a partially complete stronghold with a Source well or other valuable object or person in the center, and Lock has a limited time each round to fortify his structure before the Clockworks show up. He also has a limited amount of Source, which is expended with each structure or repair made. This much is similar to other tower defense games.
Then the building section ends, and Lock's Quest becomes something wonderful. When the building is complete and the Clockworks start showing up (from their clearly-defined entry points), Lock sticks around. The following two minutes is spent directing Lock around the field with the stylus, actually fighting the enemies. Clicking on an enemy directs Lock to attack automatically, but you can boost the attacks with a series of clever and fun stylus motions. For example, the first one you learn requires you to tap three buttons in numerical order quickly to boost an attack, then four, and then five, until the enemy is dead.
Even better, Lock has the ability to repair structures before they're destroyed, in much the same way. By tapping the structure, you can expend a bit of Source and fix a wall. There's a stylus motion to speed that up, as well, involving a "ratchet" icon at the bottom of the screen.
Now rather than being held captive and forced to watch yourself lose, you have some agency after you build your walls and set your traps. You can actively defend your towers. Maybe this is heretical among tower defense purists, but it makes the game infinitely more enjoyable to have some control over the situation. And driving the enemy forces back in one location, then running over and fixing a few walls, then taking out an archer or two on the opposite side is actually fun. It's very actiony. It's an excellent way to blend strategy and action.
In fact, there's another layer of strategy in the action as you are forced to choose high-priority enemies, often bosses, while maintaining your defenses. In the occasional Siege Mode, a side-view minigame in which you tap to shoot cannonballs at oncoming enemies, the balance tips even more toward action. This minigame is outrageously fun, and happens to be a port of 5th Cell's mobile game Siege, integrated into Lock's.
In terms of storyline and theme, Lock's Quest actually has a lot in common with 5th Cell's previous game, Drawn to Life. Both games are built around the user creating elements of the environment; both games involve a single hero protecting a peaceful town from an endless army of automata. But while the previous game was a generally easy platformer primarily for kids (though I enjoyed it!), Lock's Quest is much more of a grown-up game. It's more complex, the story is more serious, and it is of course harder.
The tutorial seems to run a bit long, due mostly to the game's insistence on slowly demonstrating stylus motions and menu choices rather than letting you do it. But by the time the tutorial was over, I was glad to have had that drilled into my head so powerfully. The game asks you to keep up with so much at once that you'll need it.
Anyone who liked the art style or the storyline in Drawn to Life will be made into a lifelong 5th Cell fan with Lock's Quest. You may even learn to like (very specific varieties of) tower defense! This game transcends genre and is worth recommending even to people who don't think they would like this kind of game.
Final score: 9/10