Roman Army Formations
The Romans used a variety of battle formations depending on the situation. For an open field conflict, the Romans would place their infantry in the middle with auxiliaries on the right and left sides. Cohorts of calvary formed the rear flanks, and there would be a large reserve of infantry men behind the main force.
Another formation was known as the pig's head. The infantry was placed into a wedge-shaped formation and would push into the enemy with a wall of shields.
When surrounded, the Romans formed a square formation with interlocking shields.
When laying siege to a fort, they used a formation called the tortoise. The soldiers in front and sides interlocked their shields. The soldiers in the back lines placed their shields over their heads to form a protective "shell" over top of the men. "It was so strong that (in training) a chariot could be driven over the top!" (Williams 2003, pg 28).