The sentine aka the sentry in the US on the c64 was a unique experience written by the legendary Geoff cranmond. As to why... 10,000 levels, strategy, 3d graphics albeit at like 2fps crammed into 32k (the original was on the BBC micro)
I've shamelessly ripped the following from Wikipedia as it's hard to describe.
In The Sentinel, the player takes the role of a Synthoid (called just "robot" in the US version), a telepathic robot who has to take control of a number of surreal, checkered landscapes of hills and valleys, by climbing from the lowest spot, where the hunt begins, to the highest platform, over which the Sentinel looms.
The Synthoid itself cannot move across the level; instead it can look around, accumulate energy by absorbing the objects that are scattered across the landscape, create stacks of boulders, generate inert Synthoid shells and transfer its consciousness from one of these clones to another.