@Trillian Thanks for including the warning. My dogs and I stay busy with people doing just that kind of thing without letting anyone know their intentions and time expected back! I may get another evening in front of the TV safe in my warm house if others followed your advice!
As for fort building, you haven’t indicated your purpose. For example, if defense of the fort is necessary, you need to incorporate that into your plan (e.g., “window” type opening to allow snowballs, rocks, etc. to fly out from the fort).
Also, go to your site for one 24 hour period and watch how the light falls at different times of day and make notes about the prevailing wind direction. This will give you some good ideas for the direction your doorway (egress) should face.
I don’t know what the weather is like where you live, but where I have built forts, it allowed for cooking on an open fire away from the fort, so I didn’t have to account for chimneys etc. Any good Boy Scout/military manual with a chapter on lashing will give you a pretty good idea of what is involved.
I like building a fort into the south side of a hill (my doorway faces south) and, as our prevailing winds come from the southwest, a small stand of trees or other windbreak just west of the doorway is beneficial. I also like being far enough up the hill that I will not be surprised by flash flooding.
I choose to use all natural materials, so I don’t bring a tarp for the roof, etc., but you could incorporate such man-made materials to your advantage.
You also need to decide where your latrine is going to be and you need to dig a hole deep enough to account for waste. I recommend a supply of biodegradable TP as well. I have lashed a perfectly satisfactory “seat” to several trees and added windbreak walls on the side of the prevailing wind for “privacy.” I keep a small shovel nearby to cover waste. You want the latrine to be upwind and downhill from the fort.