A gate tower is a tower

Tower

A tower is a tall structure, usually taller than it is wide, often by a significant margin. Towers are distinguished from masts by their lack of guy-wires....

 built over or next to a major gateway

Gateway

A gateway is a point of entry or exit at which a gate may be hung.Gateway may also refer to:-Computer terminology:*Gateway , a computer or a network that allows or controls access to another computer or network...

.

Usually it is part of a medieval fortification

Fortification

Fortifications are military constructions and buildings designed for defence in warfare and military bases. Humans have constructed defensive works for many thousands of years, in a variety of increasingly complex designs...

. This may be a town or city wall, a fortress or a castle

Castle

A castle is a type of fortified structure built in Europe and the Middle East during the Middle Ages by European nobility. Scholars debate the scope of the word castle, but usually consider it to be the private fortified residence of a lord or noble...

. The gate tower may be built as a twin tower

Twin Tower

Twin Tower is a 1970s' design of residential block of Hong Kong public housing estate. The plan looks like two hollow squares joining one corner of each square. Two towers are of different heights although their shapes are the same....

on either side of an entranceway. Even in the design of modern building complexes, gate towers may be built symbolically as a main entrance.