Shad'O is a game of hints and allusions, a game where you're never sure what's going on or what's coming up. The story of Shad'O is revealed in little snippets upon completion of a level; nothing is made too clear right away, because this game is all about revealing (or, more accurately, protecting) memories in the mind of a little boy. As you progress, more and more is revealed about the boy's story, what happened to his memory, and who he actually is. In a small way, it's quite compelling, like an After School Special with tower defense gameplay thrown in.
Like in most tower defense games, the gameplay of Shad'O is pretty straightforward: on one side of a path (or multiple paths) is the memory, your home base. On the other side is a nightmare spawner that, at regular intervals, generates bad guys with a variety of strengths and weaknesses. These bad guys move along the pathway toward your memory, and you must build and direct a series of "companions"--autonomous defense entities--on the sides of the pathway to interdict and destroy the nightmares. Different companions do different things: projectors beam out a continual shaft of light that slowly degrades enemy health within its cone of effect, while punchers slam the ground and stun enemies, slowing them and cracking open any armor they might have, for example. As you progress through the game, new companions are unlocked, adding new abilities and wrinkles to the defensive gameplay.
A successful defense of your memory relies on a good combination of these companions, placed properly, and a little bit of luck, because Shad'O disguises which types of enemies are coming at you under a cloak of shadows until they get to areas you've already placed companions in. You've also got some spells on your side, each one unlockable after a victory, along with the ability to unlock power-ups for your companions. Spells tend to cause area-wide effects--either buffs for your companions, or damage to the enemy--and are powered by killing bad guys, who drop little orange spell-power spheres when the companions destroy them.
It's all painstakingly balanced, and games play very smoothly. No map feels unfair, although they're far from easy, especially at the end. Plus, once you've beaten a level you can play it again on nightmare mode, with powered-up enemies. Beating that mode gives you another chance to unlock an upgrade, and reveals some backstory, as well. Much of the challenge of either normal or nightmare mode comes from using your relatively limited resources (generated at regular intervals by companion miners) as efficiently as possible, by choosing the most effective companions for the enemies on a given level, placing them in as complementary a fashion as you can, and using spells when you need them. It's certainly good fun, but it gets very repetitive very quickly, so Shad'O is best played in short bursts.
While you're in a game session, though, you'll appreciate Shad'O's lovely graphics and cool aesthetic that invokes the inside of a person's mind--or more accurately, his thoughts. Music is understated, although sound is critical to gameplay: because you often can't see what's coming at you, a good part of setting up your defense involves listening for telltale sound cues and acting accordingly, since most bad guys have specific sounds they make.
One thing that's not as elegantly designed, though, is the game's entirely mouse-driven control system. Sans hotkeys, Shad'O can be unnecessarily hard on the fingers. Scrolling is overly slow and annoying on the biggest maps, and would have been greatly helped if the designers had implemented hotkeys for jumping around from location to location. Another issue is the translation from the game's original French: it's often clumsy, sometimes to the point that it detracts from the storytelling, which, for a game that focuses on the drama of its backstory, can kill the mood.
Still, these are minor quibbles in the grand scheme, and they likely won't interfere with your enjoyment of Shad'O's gameplay. As tower defense games go, it does just about everything well, and while it lacks long-term depth, it does make strides to compensate for that with its unconventional storytelling.