Karel spends his life on the corners of streets and avenues that run vertically and horizontally over the screen. The street corners form a rectangular grid, whose size is chosen such that it just fits on the screen.
On some of the corners stand walls so that Karel cannot pass. On others may lie beepers that Karel can pick up. He can also mark a corner by dropping a beeper. Karel may walk over his world, but of course he should not leave the screen.