You asked: "What exactly is a 'rood'?" RE Dwelley's "Gaelic to English Dictionary", pg 764 under "rod": "way, road, path, track, ditch; perch of land. Method. Rood of mason-work. Measure of 6 lineal yards." There are 3 other categories of Rod there, one including the waves of the sea-shore.
Temples once used "screens" to shield the most holy events or relics (Ex 25:23-40; 26:31-35). Later usage had one screen which then degenerated to a symbolic thing, now its rare. However in serious temples today these are still used, probably with some knowledge of what they are really supposed to be doing (note my comment in other earlier message.) The most effective and one used with probably a full knowledge not only of the temple application but of the whole European grid dynamic, is the one at Montserrat basilica, Catalonia. There it is also used with nationalistic coding, producing a sick field around it. (Their geometry is the same used at Chartres, Rosslyn, etc. in fact the place connects with Rosslyn via the cathedral at Tarragona, in the grid I mentioned.)
Winchester Cathedral also has one that is virtually full across the choir but they also seem to know what its for. The one at Chartres was long ago destroyed in an effort to purge the Magdalene Heresy. (Many windows were also broken out in the "power bay" as part of that bright idea. My point being that windows and other energetic devices are used as the "Rood Field.")
In accord with my previous comment: that the rood screen is set on the main temple axis as a form of "band pass filter", that is, it conditions the signal in the axis (ley line) ...we note in the Gaelic definition of the "path, track, etc." that we might say that the "Rood is the Path and is in the Path" as a screen in the axis (or "path"). In this position, between the Holy of Holies and the rest of the temple, the temple's axis can have effect on the people and the process without the Holies being influenced. We insert our special "program codes" via the Rood.
I'm sure this answers all your questions(?).