Round Towers generally date to the period from the 9th to 12th centuries and probably served a variety of functions, from acting as a belfry to call monks to prayer to a refuge in times of strife. In all but one surviving case they have doors at first-floor level to accommodate either pole valuters or beard-rapelling monks, or (less likely) access by rope ladder [here | here].
Glendalough’s round tower is about 30m tall with an entrance about 3.5m above the present ground level and is constructed from mica-slate and granite. Having suffered damage in a lightning strike, its conical roof was rebuilt in the 19th century using the original stones. Internally, the tower held six wooden floors, each connected by ladder and lit by a single narrow window. The topmost floor had four windows, facing the cardinal points.
Much of the detail about the individual sites has been rather shamelessly taken from some excellent sites & I urge you to go and explore them too:
To view the 3D Images you’ll need a pair of red/blue glasses. These can be purchased relatively cheaply from Amazon [here].