Source code: http://www.mojang.com/compo/metagun/metagun-source.zip (linked from http://www.ludumdare.com/compo/ludum-dare-18/?action=preview...)

I compiled (yucky) with (after unzip + mkdir build):

    javac -cp build src/com/mojang/metagun/{entity,level,screen,}/*.java -d build/
    cp {src,build}/com/mojang/metagun/screen/exposition.txt 
    jar -cf app.jar -C build . -C res .

You need some html to run an applet; App.html just needs this:

    <applet code="com.mojang.metagun.Metagun" archive="app.jar" height="480" width="640"></applet>

Run with:

      appletviewer App.html

New to me (but probably standard to game writers) is: using a png to represent all the maps combined (like 10x10 maps), with each pixel's color representing a game tile type (eg. yellow for oil drum, purple for portal cube, red for spikes, green for signs etc). This representation is really cool, because it looks like what it represents (an isomorphism): you can "see" the level in a png editor, even though what you're seeing is not the actual level.

Another aspect of how the data representation drives the game, is that there are no explicit arcs between game screens - instead, the screen design allows your character to walk off-screen, then that point enables you to transition to the next screen. I guess it's the most obvious way to do it: it's as if you are zoomed into a particular section of the 10x10 maps, and then you can move to adjacent maps. Maybe it's just that I've spent too much time with nodes and arcs recently :-), expecting logical concepts to be explicitly specified, instead of implied by the design. Anyway, there is no sequential index to get to a particular screen; you have to follow the path. Or you can look at that png. eg, the final boss screen is at xLevel=3, yLevel=10 (you can initialize those values in GameScreen.java; and you need to change the player start location too, because now it's not derived from the previous screen: change the last two args in the following new Level line, from 0,0 to 10,10).