Initial Stage Single-Colored Walls Textured Walls Textured Floors & Ceilings First Iteration of Lighting First Version of the Portals Editor Multiple Sectors Skybox Slopes Room Over Room (Animated) Sprites & Decals First Iteration of the Particle System HUD & Font System Conversation System Immediate Mode GUI in the Portals Editor Texture Viewer in the Portals Editor Winding Markers in the Portals Editor Second Iteration of the Particle System