OK, I threw tent vs tarp out on the first page. Most have gone with tent while a rare few went with tarp.
I'll defend the lowly tarp.
Both offer shelter from bad weather. In good weather both would stay packed. In real cold clear weather both can be layed out and wrapped over your sleeping bag/cover/bed.
A tent must be set up and your equipment moved inside or stacked outside. Once inside you are stuck there. Go inside wet in a driving rain and there you are wet. Keeps raining for three days and there you are unless you pack up in the rain and move out.
Once inside your visability is limited. Hear a noise outside and you have a limited field of view and must move or make noise for a better look. Getting out in a hurry can sometimes be comical - and that would be the last thing needed in an emergency.
A tarp can be set up a great variety of ways, over a rope tied between two trees and staked to the ground on each side like an A frame tent, one pole up in a diamond shape with it's back to the wind or up high like a fly.
In a driving rain you can pick a spot drop your ruck and set the tarp up as a fly over it. High at one end would allow you to build a small fire under it with the dry tinder you keep on you. Wet wood can be added to keep it going so you can brew up some hot water for pine neddle tea.
Also with a tarp if you hear a noise at night you can just open your eyes and you have a pretty good field of view without moving.
Winter Quarters? Review the huts built at Ft Donaldson during the Civil War. Easy quick shelters with a small fire pit.
Edited to add hut link
http://www.nps.gov/fodo/photosmultimedia/tourstop3.htm
This is a big one but shows a fire place. Some were half dugouts.
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