tower -
a structure taller than its diameter; can stand alone or be attached to a larger building
structure,
construction a thing constructed; a complex entity constructed of many parts; "the structure consisted of a series of arches"; "she wore her hair in an amazing construction of whirls and ribbons"
barbacan,
barbican a tower that is part of a defensive structure (such as a castle)
tower -
anything that approximates the shape of a column or tower; "the test tube held a column of white powder"; "a tower of dust rose above the horizon"; "a thin pillar of smoke betrayed their campsite"
Towers are tall structures that are almost always taller than they are wide, usually by a significant margin. Towers are generally built to take advantage of their height, and can stand alone or as part of a larger structure.
OmegaWiki Dictionary
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tower Structure, usually taller than it is wide, often used as a lookout.