There are many synonyms for the word tower. Merriam Webster defines tower as a "building or structure typically higher than its diameter and high relative to its surroundings."
So, there are many different tall structures that could be defined as towers, and any one such small structure has various names. For instance, some small towers are turrets. A turret is defined as "a little tower."
Another synonym is a steeple or observatory (a steeple typically involves religious buildings, whereas an observatory is a tower structure meant for observing the skies). Additionally, parapets are low, wall-like structures that are used in conjunction with towers, and in some cases are themselves a small tower on a defensive structure (like a fortress).
There are many different words for a small tower, it just depends on the context.