How deep does the infantry line have to be in order to resist a charge of 15th century knights (cca 1450 - 1480)? 10th century Byzantine infantry deployed typically 7 ranks deep (2 or 3 rows of spearmen, 3 rows of bowmen, and then 1 or 2 rows of spearmen on the inside), but kataphraktoi used on the Eastern front at the time were very different from 15th century Western European heavy cavalry.
Curiously, this exact same question has come up on another forum I'm on. The Swiss did it with lines that were 8 deep, but this was towards the end of the century. A miniature by Enguerrand de Monstrelet of Agincourt shows spearmen massed at five deep.