Passage 10.21.2
τὰ δὲ ἱππικὰ ἀμφοτέροις ἀχρεῖα ἐγένετο ἅτε οὐ στενοῦ μόνον χωρίου τοῦ μετὰ τὰς Πύλας ὄντος ἀλλὰ καὶ ὑπὸ αὐτοφυοῦς πέτρας λείου καὶ διὰ τῶν ῥευμάτων τὸ συνεχὲς τὰ πλείονα καὶ ὀλισθηροῦ. τοῖς δὲ Γαλάταις τὰ μὲν τῆς σκευῆς ἀσθενέστερα ἦν---θυρεοὺς γὰρ τοὺς ἐπιχωρίους εἶχον, καὶ ἄλλο σφίσιν οὐκ ἦν ὅπλον σκέπη σώματος---, πλέον δὲ ἔτι ἐμπειρίᾳ τῇ ἐς τὰ πολεμικὰ ἀπέδεον.
The cavalry of both sides proved useless, not merely because the terrain beyond the Gates was narrow, but also because it consisted largely of smooth, exposed rock, naturally polished and slippery from the streams that continually flowed across it. The Galatians, moreover, had inferior protective equipment, carrying only their native shields for defense and having no other armor to shelter their bodies; and even more significantly, they lacked sufficient experience in warfare.