the tutorials really are more helpful than most of the information we can give, especially for the 'essential' things like what you're asking.
A heightmap is basically the vertical dimension of a landscape. If you look at any geographic map, you can see where the elevations are. Its practically that, but editable. I wouldn't mess with it, if i were you. its scary.
I don't know what you mean by Chode's abode treehouse... is this a mod?
The best tutorial for you is this, if you have the patience to follow it word for word. It took me about a week to complete (Mostly due to CS crashing all the damn time! Rawr!) but it will go through the essential stuff for you. including making doors lead places.
http://cs.elderscrolls.com/constwiki/index.php/Category:My_First_DungeonHope that helps as all I can