In 2013, Ivanzx, on his blog 'El rincón del Spectrum', started a contest to port any game from a spanish arcade machine to the ZX Spectrum. I decided to port Altair, a game from 1981 by the spanish company CIDELSA, which is a shoot 'em up with some interesting gameplay. It was finally the winner of the contest.
I released the game under the pseudonym Inmensa Bola de Manteca. When I was I child in Spain, as a joke, my friends used to say that the initials of the computer company IBM meant Inmensa Bola de Manteca (Immense Ball of Butter).
The game was programmed completely in Z80 assembly.
Later, I ported it to the Amstrad CPC.
The game is released as free software. You can download the most recent source code here: altair_src.tar.gz or browse the source repository at https://github.com/jorgicor/altair. Check the latest changes: ChangeLog (2019-01-31).
Here you can find the tape and disk images for ZX Spectrum and Amstrad CPC emulators (updated on 2019-01-31):
The original (but old) SDK release for Windows only is still available here: altair_sdk.zip. This is not updated anymore.