I want to build a small birdcage-like tower out of dark royal icing (4 walls and a rounded roof). Does anyone have a recipe for the right royal icing or tips on how to do this effectively?

Should I draw it out on parchment paper, let it dry, then tack it all together with more royal icing? That is my first instinct (using crafter's logic).

That's actually what books with templates have you do. Tape the parchment paper on top of your design paper then pipe. If you are making walls that are identical, having one drawn template paper and several parchment papers will ensure the walls come out the same size. As to recipes, I cheat and use the egg white powder in mine.