There are three camera styles available in The Witcher: two top-down perspectives, where the mouse is used to control everything, and an over-the-shoulder view, which brings the player closer to the in-game combat, but limits visibility. In all three views the controls can be changed to be primarily mouse focused or a combined keyboard and mouse approach.
The combat system in The Witcher represents a departure from most RPGs. Players choose one of three fighting styles. The quick style allows for faster, less-damaging attacks with a greater chance of hitting faster enemies; the heavy style deals more damage in exchange for a slow attack speed, and a lower chance to hit faster enemies; and the group style features sweeping attacks best used if Geralt is surrounded. The player can switch between the styles at any point. Both of Geralt's main swords also have distinctively different combat styles from other weaponry, and serve very distinct purposes. The steel blade is used to fight humans and other flesh-and-blood beings, while the silver sword is more effective against supernatural monsters and beasts (against some of which steel may have no effect whatsoever). The player can, with precise timing, link Geralt's attacks into combos to more effectively damage enemies.
Alchemy is a major part of gameplay. The player can create potions that increase health or endurance regeneration, allow Geralt to see in the dark, or provide other beneficial effects. The recipes for these potions can be learned through scrolls, or by experimentation. Once the player creates an unknown potion he can choose to drink it, but if the potion is a failure it will poison or have other harmful effects on Geralt. Each time Geralt drinks potions they increase the toxicity level of his body. This can be reduced by drinking a special potion or by meditating at an inn or fireplace. In addition to potions, the player can also create oils used to augment the damage done by weapons, or bombs as weapons in combat. Neither can be created until talent points have been allocated into the corresponding skills.
A time delayed decision-consequence system means that the repercussions of players' decisions will make themselves apparent in plot devices in later acts of the game. This prompts the players to put more critical thinking into making each decision, and circumvents a save-reload approach to decision making. It also allows the game to have a unique approach to replay value, as the consequences resulting from the player's decisions can lead to great difference in the events that take place later, and ultimately a very different gameplay experience than in prior play-throughs.
The nature of the options faced when playing the game rarely falls into the typical black-and-white morality present in most computer RPGs, and the players often find themselves choosing from the lesser of two evils rather than making a clear choice between good and evil, a situation more reflective of real life morality.
The Witcher is powered by a heavily modified version of the Aurora Engine by BioWare, enhanced for a single-player experience. A number of changes have been introduced to the original engine; some of them are described below.
One of the most important features of the Aurora Engine is that the world is designed exactly as the developers envisioned, rather than using a tile-based system. All the environments are developed in 3ds Max and then exported into the game engine. As a result, developers can create unique game worlds, rather than recycling the same tiled objects over and over again. CD Projekt's version of the engine supports lightmaps generated in 3ds Max. Shadows generated this way are reported to look more realistic, and provide better game performance.
The modified engine also includes texture paint, a special tool that allows the developer to paint the environment using custom textures. This enables the developer to make the game world truly unique. New realistic skyboxes and water effects designed specifically for The Witcher were added to the engine. The natural light during various phases of the day is realistically altered, and the day and night transitions serve to enrich the game's ambience. The weather can dynamically change from a light drizzle to a dark, stormy downpour accompanied by thunder and lightning.
All the in-game and tool set rendering is done using DirectX 9, and the engine now supports many different shaders (water effect, bump mapping, environment mapping, etc).
Other important changes include motion-captured animation, improved physics modelling, new mechanics and combat system. Additional modifications include the introduction of portals and the inclusion of additional graphical effects (glows, advanced dynamic shadows, blurs, etc.)
Source: Wikipedia, "The_Witcher_(video_game)," available under the CC-BY-SA License.