3D Monster Maze for the ZX81..utterly engrossing.
I remember getting the ZX81 magazines and typing pages and pages of code on an awful 'touch sensitive' pad (mine needed a firmer touch lol) just to have a sideways letter 'A' shoot full-stops at wave after wave of letter 'Os' and 'Ds'
Only it took days and days to type it in then it wouldn't work with a syntax error and you had to proof-read 1000s of lines of code to find out where you used a comma instead of a period....or the electricity ran out and mum had no more 10ps
....Ahh the memories...