Irish Round Tower's.

Round towers are an almost a uniquely Irish Architectural phenomena. They first appeared on monastic sites about the year 900 and continued to be built into the 1100's. It was always thought they were built as a refuge from raiding Viking's and unfriendly local rulers. This indeed would have been the case, but the sheer numbers of people attached to a monastery could not have been contained within the limited confines of a single tower.

The Irish word for then is cloigtheach meaning "bell house" and they were in fact belfrys were hand bells were rung to summon monks to prayers, and meals. They no doubt also served as a place of safe keeping for the monastic treasures, and holy relics

The remains of about sixty five of them can be found in Ireland, (some sources report over 100) some in remarkably good condition. Occasionally they are attached to other buildings, although originally they were freestanding, the average height was about 20 Metres (65 ft) some were as high as 30 Metres (100 ft).

The original design was eminently functional, their circular walls of mortared stone rising to as much as 30 meters, and tapering slightly inwards. There was usually a single door set at between two and five meters above ground level, and accessible only by a retrievable ladder. Inside there were five or more storeys each with a wooden floor and a single narrow window, with the exception of the top floor which had four windows, giving all round vision and presumably allowing the sound of the tolling bell to be heard by monks working in the fields.

See also The Round Towers of Ireland by John Healy.
Read about Irish Round Towers from 'A Hand-book of Irish Antiquities' by William F. Wakeman
Monastic sites in Ireland
The Vikings in Ireland