LostSoul said:
The only problem I might have as a player is that old medieval windows aren't the same as modern ones, and (for me) it isn't a given that you can see through them so easily (ie. concealment modifier).
Good point. We can't guarantee that windows in the original poster's world are anything like mediaeval windows*, but if they were this trick would be hard to pull off.
Windows in most mediaeval buildings of everyday occupation (ie. not cathedrals, not the palaces of the very wealthy) were simple openings that were when necessary closed with a wooden shutter to keep out wind, rain, and snow. Yes, the interiors of buildings got dark and stuffy in bad weather. Most likely mediaeval 'window' in the situation described would be covered with half-inch hardwood boards.
The next grade up, used by the middle class, was that the windows were covered with wooden frames (or even casements) over which was stretched parchment than had been made translucent by rubbing it with grease. This kept out wind and rain, and let in a diffuse glow. But it didn't retain heat very well and the window would probably have been shuttered if it were snowing outside. If the window were not covered with boards, next most likely is that it would be translucent, functioning in effect like
invisibility both ways.
Top grade would be glass windows, which would consist of small (up to 4" across, and probably smaller) panels of highly irregular "broad sheet" or "crown" glass 3/8" to 5/8" thick (thickest around "bullseye's" in the middle of each panel) set in a framework of lead strips. Such windows let in a lot of light, and keep out cold better than oiled parchment. But they would still likely have been draped at night and while it was snowing. And you can't recognise a person by looking at them through the window (unless they are right next to it), beacuse of the distorions produced by the irregular glass. In mediaeval times glass windows were fantastically expensive. John of Gaunt, a duke of Lancaster so wealthy that squabbles over his estate brought downt he kingdom of England in the 14th Century, owns a set of such windows that he
took with him as he travelled around the countryside, and fitted to the windows of his quarters whichever of his castles, manors, or palaces he happened to be staying in from time to time.
Plate glass (such as you find in old-fashioned windows with several panels set in a wooden framework) was not invented until 1620. Technology steadily improved from then, with panels getting larger, irregularities slighter, and glass less expensive. But the invention of float glass (which put single-pane window sashes within the budget of the common person) was not invented until 1959.
*For example, I estimate that a 9th-level wizard with the skill Craft (glassbowing) could use the
Fabricate spell to produce good-quality window glass (equivalent to polished plate c. 1900) for 1.3 gp/square foot wholesale. Whether this spell lets a caster make products in advance of campaign-contemporary finishing techniques is not discussed in the rules.