How to understand better the infinitive after πρίν in πρίν εχεσθαι τα άκρα οιδ’ αν ουκ εδεισθε ειρήνης. Before the heights were taken or before there was an attempt at taking the heights.
This question is answered by the previous context (3.2.10)
ὡς δὲ διώκοντες οἱ Χαλδαῖοι εἶδον ἐναντίους μαχαιροφόρους ἱεμένους ἄνω, οἱ μέν τινες αὐτοῖς πελάσαντες ταχὺ ἀπέθνῃσκον, οἱ δʼ ἔφευγον, οἱ δέ τινες καὶ ἑάλωσαν αὐτῶν, ταχὺ δὲ εἴχετο τὰ ἄκρα. ἐπεὶ δὲ τὰ ἄκρα εἶχον οἱ ἀμφὶ τὸν Κῦρον…
Xenophon. (1910). Xenophontis opera omnia, vol. 4. Medford, MA: Oxford, Clarendon Press.