A typical gate double

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Tyes of gates

The Romans were great builders. Their ruins still cover Europe, from town walls, baths, forums, arches, forts and civic buildings. 

The Romans attempted to standardize their military buildings. Here is one type of gates to a fort-the double gate. Forts could have single, double or even triple gates always between towers. 

Forts and cities would incorporate different kinds but uniformly following the different types.