The Irish round tower depicted in your question appears to be the Glendalough monestery located in County Wicklow, Ireland; founded in the 6th century by St. Kevin, a hermit priest, and destroyed in 1398 by English troops. The buildings which survive today probably date from between the 10th and 12th centuries. The leaning tower of Pisa, Italy, is considered a round tower. There are many in Scotland, Italy, Denmark, England and Ireland, and the Newport Tower in Rhode Island.

I have posted a new discussion which may bear on this. Not only were the towers in Ireland but also in Asia with the bottom more bell shaped. No one understands why there was infill to meet the door inside. Since they were not part of the existing economy, perhaps they were a 'memorial' to the past. Something to remember & as we ourselves create memorials to indoctrinate our young.