Geoff Crammond‘s The Sentinel is a strange chess-like game where you have to sneak up on an overseeing Sentinel, who is perched high on a platform, overlooking the play area.
To move you must be able to see the top of the space on the chequerboard landscape that you want to move to. And to be able to see the higher areas of ground you must absorb trees and move boulders, which you can then stack and stand on to raise yourself higher.
The ever-present risk is that you must avoid the gaze of the rotating Sentinel itself (possible by hiding behind the landscape), and its deputies (there are other sentries dotted around on some levels).
The Sentinel was a critical success when it was first released in 1986, and is seen as a classic now.
Definitely one to check out if you like games that require some thought.