E.C. Marchant, Commentary on Thucydides: Book 3

ἑλὼν οὖν . . προύχοντε—when, as in the case of Megara (I. 103), a city had long walls running down to its harbour, it was usual to prolong the walls aeross the mouth, and to leave only a small passage between two towers, forming what was called a κλῃστὸς λιμήν. ἀπὸ τῆς Νισαίας προύχοντε are to be taken together. The order is unusual, but (1) ἀπὸ τῆς N. gains prominence by its position (cf. Jebb on Soph. Antig. 325), (2) it is certainly not stranger than II. 7 πρὸς ταῖς αὐτοῦ ὑπαρχούσαις ἐξ Ἰταλίας καὶ Σικελίας τοῖς τἀκείνων ἑλομένοις, where ἐξ . . Σικελίας goes with ἑλομένοις. (If taken with ἑλών, ἀπὸ τῆς N. is rendered either (a) ‘on the side toward N.,’ or (b) ‘on the side away from N.’ But (1) in either case δύο πύργω then raises a difficulty, because they are then both on the island, and above, it was one tower: (2) neither meaning has really been proved possible: in the passages quoted for (b) ἀπό=‘at a distance from’ with a verb of rest. If not taken with προύχοντε, ἀπὸ τῆς N. would naturally=‘starting from N.,’ with N. for a base; (3) with (a) it is impossible to explain καὶ τὸ ἐκ τῆς ἠπείρου, which must denote a different place from ἀπὸ τῆς N.)

ἐς τὸ μεταξὺ τῆς νήσου—supply καὶ τῆς ἠπείρου, as in Dem. de Cor. 26 τὸν μεταξὺ χρόνεν τῶν ὅρκων, ‘the interval between (that time, and) the oaths,’ Aristoph. Av. 187 ἐν μέσῳ δήπουθεν ἀήρ ἐστι γῆς, and often. Sometimes the other limit is expressed. (It might mean ‘to the part of the island lving between,’ as Isocr. IV. 70 ἐν τῷ μεταξὺ τῆς χώρας, but on the whole this seems less likely.)

ἀπετείχιζε καὶ τὸ ἐκ τῆς ἠ.—‘he proceeded to build a wall on the side facing the mainland as well,’ apparently at the island end of the bridge. The καί refers to πρῶτον.