“Why is there a wooden beam mounted inside the round towers?” With this question has usually come a suggested answer such as “to support the wall”, “for a floor”, or “ a safety rail or walkway”. What they actually do is serve as a guide in the construction of the tower. They are huge compasses. Rather than a single beam, they have two parts: a beam from wall to wall and an upper beam made to rotate off a point on the first beam that is the center of the tower. It’s the biggest compass I’ve ever seen.
This is another example of the close cooperation between workers of different skills at the Ozark Medieval Fortress. It is also very accurate to life and work in the Middle Ages. The carpenters assembled the wooden compass beams, the blacksmith provided the pivot pin and pins to hold it into the stone wall, and the rope maker provided the ropes that strengthen the upper (rotating) beam and the masons, of course, installed it and use it to keep the tower round and true. The way the mason does this is to rotate the compass (the upper beam) and align the ascending tower wall to the end of the beam using a plumb bob. This means that the tower wall remains round and vertical. A plumb bob is a weight on a string that, because of gravity, hangs straight down and was an important Medieval tool. Incidentally, the bob or weight was usually lead. The word “plumb” comes from the word “lead” and because early pipes were lead, we still call pipe-workers plumbers.
The masons are able to use the compass not only to keep the inside wall true, but also the outer wall by simply measuring from the inside of the wall to the outside with a stick and keeping the width constant. They also use their plumb bob on the outside of the wall to keep it vertical. Every rock has to be checked for plumb as they go to avoid magnifying an error. You can’t have a wavy wall on a 45-70 foot high tower!