The ZX Spectrum is an iconic machine, it hung around for over 10 years and over 9000 games were made for it. R-Type is a legendary game on the machine because it was such a remarkable achievement on 48k! To someone unfamiliar with the Spectrum it won't look like much, but you have to remember that the Spectrum had no hardware-sprites, no hardware scrolling, only fifteen colours and only allowed two colours per 8x8 pixel square.
Bob Pape, the programmer of the amazing R-Type conversion on the ZX Spectrum has written a free e-book about the game making process. The book is self-published and available to download online. I haven't read it all the way through yet but what I have read is very interesting. The retro UK shooter fans will know how legendary this conversion was back in the day, I being an Amstrad man had that version but retrospectively played the Spectrum version and man is it good. So read the book people!