Open Source C Role-Playing Games (RPG) for Windows

Haxima is an Ultima-like game engine (2D tile graphics) with combat, magic, NPC AI, an embedded scripting language, and in-game editing.

IRE is a tile-based Computer Role-Playing Game engine based on Ultima 6, but with an emphasis on customisation and cross-platform support. Everything is controlled using scripts.

MAP is a software that allows to create and play text adventures. Text adventure or "interactive fiction" is software simulating environments in which players use text commands to control characters and influence the environment. They were among the first computer games. MAP includes tools to create the adventure and a runner that allows to play it. Source is available and open so both the runner and tools can be modified to fit the needs or style for each adventure/creator.

Action/RPG Game Engine. Overall game engine is to be open source, but not the art assets.