There is a quite detailed exploration of the Newport Tower at:
http://www.neara.org/CARLSON/newporttower.htm
If you have some time to read it all...
The common thread in a loose web of structures stretched across Europe, disregarded by architectural historians, is found in the polygonal, usually octagonal, arcaded open enclosures surmounted by a superstructure also round or polygonal, difficult of access and displaying local tastes in architectural style. Whether called more romantically from the Latin “lavabos” or just wash houses ....
But we can be sure that the trade secrets of the builders were shared. We can guess that they were also shared with their spiritual brothers, the Templars and with a legacy imparted down through the generations of initiates from Scotland to Portugal they were united in their vision.
The question remains—who built the Newport tower, when and why?