As a programmer, you probably want to jump straight into the action, so take a look at the examples in examples/ subdirectory of the engine. You can compile them from Lazarus (install castle_base and castle_components packages first, and then just open examples .lpi files), or by make examples.
There are a lot of examples, for 3D game examples look e.g. at
Compile programs using the xxx_compile.sh script prepared in directory of each program. In summary, this script just calls fpc with proper command-line options. Or open the xxx.lpi file in Lazarus, and compile from Lazarus.
To use the engine in your own programs, there are a couple of options:
If you use Lazarus, the most comfortable way to use engine units is to use Lazarus castle_base and castle_components packages. Just install them in Lazarus. See castle_game_engine/packages/README.txt in sources for more information.
If you don't use Lazarus, you should compile engine units using castle_game_engine/Makefile file. GNU make is required (under Linux this is the default `make', under Windows you have this already bundled with FPC (you can also get this from Cygwin or MinGW), under FreeBSD this is called `gmake').
If you need more control over compilation process then you can also directly use castle-fpc.cfg file. This is my configuration file for FPC. It's used by and by all xxx_compile.sh scripts, fpmake.pp etc. All my programs and units must be compiled with FPC using this configuration.
libpng and zlib.
CastleImages unit requires libpng (libpng.so under UNIXes, libpng12.dll under Windows) installed if you want to load/save PNG images. Otherwise you will get ELibPngNotAvailable exception when trying to load/save files in PNG format. Note that libpng depends on zlib.
CastleZLib unit (required by CastleZStream unit and used by X3DNodes) requires zlib installed. This is used to open and save gzip-compressed files.
To play 3D sound. If OpenAL is not installed, everything will stil run just fine (we will not raise any exception), you will just not get any sound.
To load sounds in OggVorbis format.
GtkGLExt (on Unix).
Used by CastleWindow on Unix to create a window with OpenGL context. There is an alternative CastleWindow backend that doesn't use GtkGLExt (uses directly Xlib), although it doesn't show native manu or dialog boxes.
Here's a short map of the important units documented here. Units are divided into a couple of groups. Group's name (like "base") is also the name of subdirectory where unit's source code is placed. The "Uses:" notes below say which units are allowed to be used by units in specific group.
Basic utilities and classes.
Uses: Nothing. These units do not depend on units from other groups.
3D sound support based on OpenAL, processing of sound files.
Image, textures, videos processing. Not OpenGL-specific.
Font support, including some embedded fonts in CastleOutlineFont_* and CastleBitmapFont_* units. Not OpenGL-specific.
A Lazarus component, by our TCastleControl class (see components directory). This descends from the base TOpenGLControl provided by Lazarus, inheriting it's functionality.
Container is tied to a particular windowing library, like GTK or WinAPI or something higher-level like Lazarus LCL. Container is responsible for initializing and managing OpenGL context.
Container handles a list of controls, that is a list of instances of the TUIControl class. For our two current containers this list is TCastleWindow.Controls and TCastleControl.Controls. Container passes to them key events, mouse events and much more.
"Controls" are classes that descend from TUIControl class. They may be tied to a particular rendering library, in practice — all current controls use OpenGL rendering.
The controls are not concerned with how you initialized the OpenGL context, so all controls are useful with all containers. For example you can use the same TCastleOnScreenMenu class inside TCastleWindow, or inside Lazarus TCastleControl component, etc.
What is important is that no other engine units may depend on CastleWindow units. So every OpenGL code outside window directory is usable with OpenGL context initialized in any way, like with Lazarus OpenGL control.
VRML/X3D rendering and processing. This is the core of our engine rendering, classes to handle and render 3D stuff are here.
CastleScene unit provides the complete class for dealing with 3D models and displaying them in OpenGL. CastlePrecalculatedAnimation also provides a special way of animating models by interpolating between a set of 3D files.