Each "place" in the world is called "node" in all the source code, in all the files, etc ...
Edit the data.xml file with a basic text editor (notepad). You can see that XML looks like HTML.
At the beginning, there is no sound, no variables, and just one node. The nodes are defined between <nodes> et </nodes> markers. You will create a second node. Replace the end of the file :
<node0>
</node>
</nodes>
by :
<node0 north="1">
</node>
<node1 south="0">
</node>
</nodes>
Remark 1 : the spaces and tabulations are not important, and are just here to clarify the code.
Remark 2 : warning : don't forget the inverted comma ! It could create a lot of incomprehensible errors.
Remark 3 : as you can see, the nodes are numbered from 0.
Launch the game. You should have created a link between two nodes : when you click on the north with the left mouse button, you should be moved in the other node (you can wait one second the node loading). Then, do a U-turn (to the south), and click : you should be in the initial node.
How does it works ? In fact, you create a second node in the xml file, and you wrote that when we go to the north from node 0, we are moved to node 1, and when we go from node 1 to the south, we are moved to node 0.
You could have written :
<node0 north="1" west="0" south="0" east="0">
</node>
<node1 north="1" west="1" south="0" east="1">
</node>
</nodes>
</body>
it is the same result.
Now you can do a lot of node using the same system, and links them. Remark : there are no links to the top or the the bottom !