Jose Javier Garcia Aranda has written 8BP (8bits de poder : 8bits of power), a RSX library for writing Amstrad CPC games in basic (26 Kb max). You can download 8bp on github (sources, demos and games examples, spanish and english documentation). It already exists three games including the last one Niburu :

• mutante montoya (platforms /maze)
• anunnaki (space ship arcade, vertical scroll)
• nibiru (space ship arcade, horizontal scroll)