Testudo was a defensive tactic used by Roman legionaries to defend themselves against archer fire. It consisted in creating a compact rectangular formation in which legionaries (usually 27) from the first row and sides of the formation held their shields vertically in front of them or from their exposed side, while legionaries from the inner ranks held their shields horizontally above each other and over the legionaries of the first and side ranks, thus creating a cover for the entire formation against enemy arrows. The legionaries moving in such a column during the battle resembled a shelled turtle – hence the name.
The principle of formation is perfectly shown to us by Plutarch:
On the following day they [Romans] went forward under better protection; and the Parthians met with a great surprise when they attacked them. For they thought they were riding up for plunder and booty, not battle, and when they encountered many missiles and saw that the Romans were fresh and vigorous and eager for the fray, they were once more tired of the struggle. 2 However, as the Romans were descending some steep hills, the Parthians attacked them and shot at them as they slowly moved along. Then the shield-bearers wheeled about, enclosing the lighter armed troops within their ranks, while they themselves dropped on one knee and held their shields out before them. The second rank held their shields out over the heads of the first, and the next rank likewise. The resulting appearance is very like that of a roof,41 affords a striking spectacle, and is the most effective of protections against arrows, which glide off from it. 3 The Parthians, however, thinking that the Romans dropping on one knee was a sign of fatigue and exhaustion, laid aside their bows, grasped their spears by the middle and came to close quarters. But the Romans, with a full battle cry, suddenly sprang up, and thrusting with their javelins slew the foremost of the Parthians and put all the rest to rout. This happened also on the following days as the Romans, little by little, proceeded on their way.
– Plutarch, Antony, 45
A well-used “turtle” was an excellent shield against ranged units, and the soldiers could move around the battlefield without fear. So surely this tactic could be effective especially in sieges when legionaries could take refuge from wall projectiles.
This formation, however, fared much worse in the field. The weaknesses of the Roman formation were proved by the battle of Carrhae in 53 BCE. The Romans, while in regular formation, were fired upon by Parthian mounted archers, and when they wished to enter the testudo formation, they were attacked by cataphracts. It is also worth noting that the soldiers from the first lines covered the entire body, but left their faces and legs exposed (the Romans did not wear greaves), which made it possible to hit them. In addition, the “turtle” was an extremely slow formation, making it an excellent target for throwing machines such as onagers and ballistae. An accurate heavy missile could kill up to a dozen legionaries.
Disadvantages of testudo formation
Apparently, the Romans tested the endurance of testudo by throwing chariots at him. You can read about such checking of formation in Peter Connolly’s book “So Lived People – History of the Roman Army” (p. 63).
Caius Dion, in turn, mentions in “Roman history” that the testudo formation was so strong, tight and resilient that it was said to be able to withstand the weight of a horse walking on shields, and even a towed cart.