τὰ (I got that the τὰ was related to οἰκοδομηθέντα ) γὰρ Θεμιστοκλέους . Didn’t know how Themistocles was declined. μετὰ τὴν ἀναχώρησιν οἰκοδομηθέντα (referred back to the ‘walls’ of prededing sentence) τὴν Μήδων Was surprised that οἰκοδομηθέντα could sit in the middle of τὴν ἀναχώρησιν τὴν Μήδων like that ἐπὶ τῆς ἀρχῆς ( I was confused by ἀρχῆς which I kept thinking of as “origin”, or “beginning”, or “ruler” which didn’t fit for a different reason) καθῃρέθη τῶν τριάκοντα ὀνομαζομένων. Meaning:”Themistocles walls, constructed after the Persian retreat, were destroyed under the government of the Thirty.”