Anti-Cavalry Barrier....

[Tarbicus]

From this paper: The Fulcum, the Late Roman and Byzantine Testudo: the Germanization of Roman Infantry Tactics? by Philip Rance

Maurice also describes Roman infantry forming a foËlkon when confronting an enemy cavalry charge, though these different tactical circumstances required certain modifications (12.A.7.49–60): ....

If the enemy [cavalry], coming within a bow shot, attempts to break or dislodge the phalanx, which is hazardous for them, then the infantry close up in the regular manner. And the first, second, and third man in each file are to form themselves into a foËlkon, that is, one shield upon another, and having thrust their spears straight forward beyond their shields, fix them firmly in the ground, so that those who dare to come close to them will readily be impaled....