The tarball you can download below does not contain the source code. It contains a static binary that should run out of the box on every x86 GNU/Linux distribution and does not require any extra stuff or compilation. When you want the source, you can download it via CVS or see further below.
Here are source code releases of Advent, note that the binaries are not always in sync with the source releases, so it can happen that some binary releases come without source and that some source releases come without a binary release. Get the source from CVS then or compile it yourself.
The .xcf source for all the images which are/will be used in Retriever, is located here
You can get the current CVS version of Advent, along with ClanLib and Hermes, like this:
$ cvs login
(there is no password, just hit return)
$ cvs -z3 co Libs/Hermes-1.3
$ cvs -z3 co Libs/ClanLib-0.5
$ cvs -z3 co Games/Advent
Then compile the packages in the order listed above (Hermes, ClanLib, Advent). You might also need to install libjpeg and libpng, they should be available prepackage in your GNU/Linux distrubtion.
You also need Guile and Goops. When you use Debian sid/unstable a simple apt-get install libgoops5-dev should do it, for other distributions you might need to compile it yourself which might be a bit tricky.