I had a 3D Tetris style game several years ago on tape called "Welltris". The game was nothing like what you have done, although it was quite good and clever. TetrISO is a very clever idea which makes Tetris more challenging. I have not seen anything like this on the C64, but I may have seen it before but as vector graphics. What you have done here is a stroke of genius. Well done.
Thank you Richard for your very kind words. I always wanted to make something in isometric style and this came out of it. Tetris is a wonderful game concept because you don't need to design any levels - just a random number generator. The basic mechanics speak for themselves. But I am quite happy how my design implementation worked out. For example, it was important to me that you can turn or push a block into position in the very last moment. And being able to turn the whole tower is helping a lot to deal with the (sometimes a bit difficult) isometric perspective. All in all, this was a nice little project. Happy that you like it.