The round towers of Ireland were built between the 9th & 12th centuries, generally next to monasteries or other religious buildings. Some are up to 40M high. The doors were constructed in an elevated position to enable the monks or priest to take their chalices and other valuables to the safety of the tower by ladder when there was a threat from invaders. The ladder was then pulled up and the door locked securely. Once the invaders departed the monks would then reappear and resume normal duties.
Many of the towers survive to this day due to their aerodynamic curved shape which prevents storm damage. The tower featured below (on a foggy day) is in Lismore , County Waterford in Ireland’s south east.