Hobnails, actually. They also calculated as a European size 40, which translate to an American size 9.5.
Caligae: Heavy-soled boots worn by Roman soldiers of all ranks up to and including Centurion. Caligae resemble sandals, but were actually designed for both marching and fighting. The open design allowed for the passage of air to the feet to reduce the likelihood of blisters forming while marching, as well as other conditions such as tinea and what is today called trench foot. Socks were not normally worn with caligae, although in colder climates (such as in Britain), woollen socks were often used. Caligae incorporated three layers of leather, with the top one forming the outer shell. These boots were laced up the centre of the foot and onto the top of the ankle. Iron hobnails were hammered into the soles of the boots to provide grip, reinforcement and act as a weapon (allowing the soldier to injure an enemy through a stamping action).
I wonder how their feet felt after a days march in those things. Yike.