Yeah the BASIC was a bit lethargic but I wrote a few games entirely in BASIC (a version of Snake was one, a turn based graphical exploration game (viewed from above with visual field calculated) was another. I also did a very large maze game, no monsters but it was 3D! (although to be fair that's pretty easy to do). But still if speed mattered there was always the USR function to call into some machine code. I tried to write a couple of more advanced games (Lunar Lander for one) using a mixture of assembly and BASIC but the assembly defeated me. The ratio of instruction count to work done was just too high and when it went wrong the machine froze or crashed and the frustration factor was too great.