Even in castles made mostly of fieldstone, dressed stone had to be used for anything structural. We can see it in the interior connecting doors that often have arches, but they’re even more key in the places where thick stone walls needed specialized hollow places.
Holes in the outside walls were a vulnerability, so they were made as narrow as possible. At the outside face, dressed stone surrounded a one-inch slit for archers; as time passed, these slits were often improved with cross-slits to allow archers to aim left or right, or with rounded holes at top and bottom. But they were never wider than absolutely necessary. However, on the inside, they had to open up immediately into space large enough for a man to sit. A four-foot wall’s outside stone contained the slit and was angled away from the slit, on the interior side. The next foot of stone had to be made wider still, and so on. Sometimes these stone were cut at angles so that the archer faced into a smooth, narrowing funnel, while in other castles, the stones opened out in steps. There was usually a stone bench for an archer to sit on, and of course the whole thing had to be tall enough to accommodate his bow.
The dark interior hallways needed as much daylight as possible, but that wasn’t much. Castle windows were usually about four inches wide, that is, narrow enough to present a difficult target for attacking archers who weren’t Luke Skywalker. As with archery slits, they had to start opening up on the interior, to take in as many hours of the moving sun as possible. One way to maximize the incoming light while minimizing the target size was to place two four-inch window slits in one window niche. (Image below is an illustration from Castles: Their Construction and History, by Sidney Toy.)
In a few castles, the mason architects planned the stonework that framed the window niche to let light shine down through an angled shaft. Nobody could peek out those windows, but if an arrow came in the slit, it hit the stone shaft and dropped harmlessly to the floor below.
Both of these holes in the wall were simple compared to chimney flues. Once chimney technology came into use, castles could be much better heated, but they required serious planning. Thick walls had to be built up in layers with the flue hole carried at an angle upward. In simple situations, it went up to the roof, but sometimes the fireplaces had to go up several stories, meeting other flues along the way. Both fireplaces and chimney flue were larger than we picture; a medieval fireplace was like a modern breakfast nook, and flues often had iron bars to act as ladder rungs. That’s how chimneys could be cleaned and maintained.