This is unquestionably the greatest puzzle game I have ever played and probably even one of the greatest games in general.

It almost never tells you explicitly what to do, opting instead to be more respectful of the player's ability to explore and experiment to find solutions and secrets on their own.

Gentle music serves as a background to a solitary journey through more than one hundred puzzle rooms carved into photorealistic ruins. It is not a funny game like Portal or an immersive game like Amnesia, it is a quiet and meditative experience.

The conditions to solve most of the puzzles are quite flexible and often have multiple solutions or can even be beaten by "breaking the rules". This gives the game great replayability even after you have solved a puzzle the first time.

The plot is basically optional--if can ignore a beeping terminal as you pass by. If you do choose to interact with the terminals you will find an interesting philisophical story that questions your beliefs on being. Because your responses to questions are limited to options in a list the edges of the dialogue tree become very apparent in subsequent play-throughs but at least the first time through the game it feels like you have a lot of agency in your responses.

The Road to Gehenna DLC is also excellent and highly recommended as it adds puzzles of greater size and complexity than typically found in the base game.

The Serious DLC is also good for a laugh on your repeat playthroughs (don't ever enable it for your first playthrough) but is probably only worth it for most to get while on sale.