I suspect Master Twisthorn was either a mage of some degree, or employed the services of one. One-way windows are a common enchantment used for scrying purposes. They tend to be fixed to a specific point in space (and sometimes time, for the nostalgic or historically inclined). I myself find them a rather trivial bit of magic, and have several in my apartments.
—Ouster Brownden, D. Univ, D.W, D.B.P., Scholar of Rheve Library, Fellow of the Order of the Jollux