Shad'O

Shad'O is a tower defense strategy game played inside the mind of a young boy called William dressed in an animal costume. The waves of monsters the player has to defeat represent his deepest fears. Their fog attempts attempts to push back the few fragments he remembers. When he manages to drive them away, flashes come back to him and by going to through the game William attempts to piece them back together to find out what has happened to him. In his mind he receives help from his teddy bear who provides hints, as that is the first thing he remembers. The level map consists of different beds that float in a sea of fog.

To win a level William generally has to protect his memory, a large light source that drives the darkness away, against a certain amount of waves. At the start of a level most of the environment is usually shrouded by fog, hiding enemy movement from view. One or more red pools appear where enemies spawn and a bar near the bottom of the screen shows the progress. To deploy units, light has to be collected as a resource. This is done through collectors, who can only be placed near a luminous well to drain the infinite supply in short bursts. When a sufficient amount has been gathered, different types of units can be deployed, introduced gradually. These need to be placed along the paths enemy creatures follow. Initially not much room is available due to the fog, but by expanding from different locations a large part of the map is eventually revealed. After placing them, units act on their own.

The initially available units include a shooter (regular projectiles), a projector (a constant beam), and a puncher (causes physical damage and slows enemies down). Later on additional ones are introduced such as the mole who can be moved to any position by digging, doc who heals nearby units, rainbow who throws a bouncing rainbow project and more, with ten units in total. There is no limit to the amount of units that can be placed on the map and there is an endless supply of light. By completing levels there is an opportunity to unlock two tiers of upgrades for each unit. These can then be activated in-game with additional light per unit to make them stronger and more effective. Enemies generally appear from one or more pools and more advanced levels have multiple paths they can follow. When a wrong strategy has been set up along the wrong path, units can be sold quickly to recover a part of the currency. Enemies all go after William's memory, but can damage the units along the way as well. The memory has a small amount of defense to finish off units that are almost defeated. When the memory takes a certain amount of hits, it is blown up and the level needs to be restarted.

Instead of unit upgrades the player can also opt to unlock a spell instead. Defeated enemies leave behind pieces of shadow that are collected by moving the mouse cursor over them. With a sufficient amount stored, different types of spells can be cast. There are spells that dispel negative status effects, clear away the fog for ten seconds, cause physical damage to opponents, boost resources for a while or provide additional ones, increase the memory's damage, freeze enemies, quickly exchange health for resources at a large scale, or restore health. These often help when the enemy is about to move past the defenses. William appears in the map to launch spells and near the memory, but he cannot be controlled or attacked and he does not interfere.

There are three difficulty levels: easy, normal and nightmare. While playing, a button in the top right corner can be used to speed up wave movement. Next to the regular levels there are separate challenges that often radically change the regular gameplay. By completing these, permanent perks can be earned. An example is a level with strong defenses available right away, but due to limited energy only one unit can be activated at a single time and no additional ones can be built. There are also boss levels that require a very different strategy. Between levels short sequences further the story.