Why were ancient Roman forts successful?

Ancient Roman forts were remarkable feats of the engineers that were part of the legions. They were experts at fortifications and innovations as time progressed.

Advanced forts with a good amount of stone walls and towers were formidable bases that enemies had problems overcoming. The entrances of stone forts were always defended by towers, which with some having towers four stories high. It should be said at this point that the Roman fortress bases were primarily a starting point for attacking an enemy.

Some unique fortress defenses were constructed with a double entrance. This concealed at times the main gateways, confusing any potential attackers. The fortress towers were built strong enough to handle light artillery weapons such as the scorpion bolt shooters.

Ditches built in front of some forts were deep and contained sharpened stakes to destroy part of an attackers forces. The entire system of fortifications surrounding a fort were designed to slow down an attacking force, and thus allow for a counter attack from one of the hidden gateways.