Jonathan Cauldwell informed me (thanks!), that he is currently porting his Arcade Game Designer to the Amstrad CPC. With the AGD you can design your own small arcade games. It features an own simplistic language, a sprite editor, a screen editor, etc. It will also be possible to port games between the different platforms, so porting back Spectrum games to the CPC ? If you want to get more information about the AGD then have a look at the AGD forums.
Here are some preview screenshots: