I played it on the speccy too. It looked incredible when you read that it had 10000 levels. And also recalled that after a month or so all the codes were appearing on the magazines at the time. Beautiful absorbing game.
The Wikipedia article states, “The Sentinel has no ending sequence; upon completion of the last level, the player is looped back to level 1. When questioned about this, Crammond said he never thought anyone would go so far as to finish the game”
That’s a fun reward!