CPCTelera is a multi platform (Windows with Cygwin, OS X and Linux) development framework for creating Amstrad CPC games in C and also assembly programmers. For all features check CPCWiki, for short :

• A low-level library with support for: graphics, audio, keyboard, firmware, strings, video hardware manipulation and memory management
• An API for developing games and software in C and Assembler
• A complete multi-platform building system with support for building CDTs and DSKs automatically. Tools for content authoring (audio, graphics and level editing)

Project creation, management and integrated build system with tools (SDCC compilator...)

autoring tools and command line tools for format conversions (Arkos Tracker, RGAS...)

Retro Game Asset Studio (RGAS) by Lachlank is the next evolution of Amsprite (now defunct). Using .NET, you can create graphics, sound/music levels for Amstrad CPC, ZX Spectrum and Commodore 64.