The Ramparts counts as a building.
upgrade => Ramparts with Flanking Towers
This building is an improvement of the Fence. It therefore requires the latter to be built.
This building requires 3000 man-days
The construction of the building requires 1 political point, 1,000 gold and 100 wood.
With the Ramparts present, the attackers must have more regiments to conquer a city: 350 attackers for cities with fences, compared to 150 attackers for cities without.
The presence of ramparts allows to get two (2) allied regiments in the city instead of an one (1).
The Ramparts lift the garrison of the city by default to 100 men (instead of 50 previously).