A first/third-person puzzle game with beautifully themed environments and captivating original soundtrack.
You solve world puzzles to collect Sigils (Tetris pieces, essentially), and you use those collected Sigils in Sigil puzzle locks (which unlock doors or new puzzle mechanics.) To solve a lock you must arrange the Sigils in a preset rectangular pattern. Along with Sigils there are collectable stars which are used to unlock more difficult content. The stars themselves are often fairly difficult to collect, but are very satisfying.
The game plays at your own pace, as puzzles are solved with theory and execution as opposed to timing and platforming. Due to the design of puzzles being self-contained experiences, you can play casually, only completing a few puzzles during a session without worrying about losing progress and having to redo a bunch of them. You can't manually save in the middle of a puzzle, but progress is saved as events get triggered, and your last puzzle/world location is kept track of, so it's easy to continue where you left off.
The game does not hold your hand as you play... you aren't told how to use puzzle mechanics, but rather you learn how to use them through natural play and experimentation. The learning curve is very gradual but you will find yourself dealing with fairly complex puzzles later on in the game as you become accustomed to the mechanics. The game isn't a linear progression of puzzles so if you get stuck on a puzzle you can skip it and come back to it later (and often solving it after having an epiphany.) The only barriers are Sigil Locks, which do require you to collect at least certain "easy" Sigils before advancing too far.
The story of the game is conveyed through various means... through ELOHIM (the voice in the sky), audiologs (from a female scientist), journal entries (archived data in computer terminals), and messages written on the walls (by various characters.) A big, unique part of the story is done through interactive events in the terminals, where you will interact with an AI character through dialogue trees. The AI will make you think, articulate your thoughts, and ultimately question yourself. If you wish to partake in the story and get the most of it, you'll find yourself having a lot to read, but it's all completely optional and can be skipped if you really just want to solve puzzles. The writing and voice acting are all super high-quality with perfect execution.
The level design is based on real-life environments and themes primarily from Roman, Egyptian, and Medieval architectures, with a few "fantasy" environments thrown in. Weather and time of day add a lot of variety to the worlds and keep them fresh and feeling unique from one another. Environmental textures and models are very detailed and are made from real life photographs and 3D scans, as I understand it. Puzzle mechanics clearly stand out from the detailed environments by being pristine and more technologically advanced in nature, as to not get in the way of solving the actual puzzles. Puzzle areas are designed to feel open and allow you to see to other areas so you get a good sense of what all you have to work with and the area you have to utilize. Areas such as walls, fences and stairs feel natural while still retaining very distinct purposes, often aiding you in coming up with the solution without you necessarily realizing it.
There's a ton more I could say about this game, but all in all you're looking at around 20 hours or more of content spanning around 120 puzzles (not including Sigil lock puzzles or additional secrets.) A second play-through is likely if you're wanting to explore alternative dialogue trees, get all the achievements, find all the secrets/messages, or attempt speed-running. There is also a level editor included as well as Steam Workshop support for user-generated content. The PC version of this game has more settings than you have ever seen before, so it can run smoothly on rather low-end hardware, and you can adjust to your liking.
Full disclosure: I was a volunteer beta-tester for this game for ~3 months prior to release, and I have ~180 hours of play in ~4 playthroughs. I highly recommend this game and supporting the developer, Croteam!