Here are the data structures with brief descriptions:

CANIMATION	Internal representation of an animation. Do not call directly; use animate_create() instead
CCONTROL_DATA	A piece of data attached to CONTROL::data
CCROP_DEF	Definition of a crop to be stored with the TILEMAP on which the crop may be grown
CCROP_PLOT
CDIO_BUFFER
CDIO_STREAM
CDSEKAI_STATE	General/shared state of the running engine in memory
CEDITOR_STATE
CFAKE_MEMORY_HANDLE	An emulated memory handle for modern systems. Overridden on most platforms
CGRAPHICS_ARGS	Platform-overridden struct supporting graphics engine
CGRAPHICS_BITMAP	Struct representing a bitmap on the current platform
CGRAPHICS_BITMAP_BASE	Struct representing a bitmap. Should be overridden by platform
CGRAPHICS_RECT	Struct representing a graphical rectangle
CHEADPACK_DEF
CITEM	An instance of an item in the world
CMENU_STATE
CMOBILE	A moving/interactive object in the world
CPATHFIND_NODE
CPOV_RAY
CPOV_STATE	State for ::ENGINE_TYPE_POV. Displays a first-person 3D view of the world using raycasting
CSCRIPT	Contains immutable state for scripts attached to a TILEMAP
CSCRIPT_COMPILE_STATE
CSCRIPT_STEP	Contains a single instruction in a script, in SCRIPT::steps
CTILEMAP	Information pertaining to in-game world currently loaded
CTILEMAP_COORDS	X/Y coordinates of a tile on the current map
CTILEMAP_SPAWN	Defines a spawner to create mobiles in the world
CTILESET_TILE	Tile prototype stored in TILEMAP::tileset
CTITLE_STATE	State for ::ENGINE_TYPE_NONE. A simple title screen engine
CTOPDOWN_STATE	State for ::ENGINE_TYPE_TOPDOWN. Displays a top-down 2-dimensional view of the world
Cunimake_state
CWINDOW	Struct representing an on-screen graphical window
CWINDOW_FRAME	Defines a graphical window frame/background with a set of image assets