The Roman military had one huge advantage over their enemies: persistance. The military would almost never back down if there was chance of victory. This was shown most clearly during the siege campaigns on enemy forts and castles. When faced with attacking a fort that was rumored to have enough food to last 10 years, a Roman centurion was quoted as saying: "Then we will take them on the eleventh year." To facilitate these long battles and sieges, there were special plans, commissioned by Rome, that were to be followed in building a fort to protect and give soldiers something to fall back on.
Not only did these forts have the standard deep ditch, but many other safeties were made to protect its perimeter. In addition to the ditch there would often be a moat, slightly deeper than the ditch and on the far side of the ditch, several thick walls made of stones and wood planks designed especially for preventing battering rams and cavalry mounted enemies from getting even close to the fort. If the battle took place in an area of high grass, Roman would seed areas with their equivalent of land mines, sharp metal or wooden spikes that projected only inches out of the soil. Very hard to detect by eye, these would hinder any enemies from marching into the fort.
These war forts themselves were built for combat. These forts were built on the general square or rectangle formula, and so at each corner and at tactical points in the sides towers would be built. These towers, often over 4 stories high, were homes for heavy artillery and archers. These towers would provide automatic high-ground for the Romans and be a major advantage if the enemy was slow moving or organized in tight formations.
These forts were often big enough to hold an entire legion, and some only large enough to hold a couple of centuries, but big or small these forts gave the Romans the edge they needed to conquer the world.