In Ancient Roman warfare, the testudo or tortoise formation was a formation utilized commonly by the Roman Legions during battles, particularly sieges. (Testudo is the Latin word for "tortoise".) In the testudo, the men would close up all gaps between each other and grab their shields at the sides (rather than through the regular straps). The first row of men, possibly excluding the men on the flanks, would place their shields in front of them, from about their shins to the middle of their faces, so as to cover the formation's front. Everybody in the middle would place their shields over their head to protect from above, balancing the shields on their helmets and overlapping them. If necessary, the legionaries on the sides and rear of the formation could stand sideways or backwards with shields held as the front row's, so as to protect the formation's sides and rear, but the shape of the shields would mean that these soldiers would only be afforded incomplete protection.

When used correctly, the testudo was an excellent shield against missile troops, and the legions could move with little fear of being slaughtered by arrow fire and javelins. The primary problem with the formation was that it was so tight that the soldiers had great difficulty fighting in hand-to-hand combat—the Battle of Carrhae showed testudo's limitations, as the Parthians shot the Romans with horse archers if they stayed in regular formation, and charged with cataphracts if they tried to form a testudo. Other problems were that the front rank's faces and legs were still exposed; the formation couldn't move very quickly; and more powerful weapons (such as Eastern composite bows) could puncture the scutum and pin the soldiers' hands to their shields under prolonged fire, as occurred at Carrhae.