προπαρεβάλοντο—had previously laid in a line for use, i.e. for the wall from the circle to Trogilus
σφίσιν—for σφίσιν αὐτοῖς or ἑαυτοῖς. Cf. on c. 3.1. Thuc. often uses σφᾶς thus as a primary reflexive, referring to the subj. of its own clause; but not when the reflexive is emphatic or in antithesis.
τοῦ τειχίσματος—the cross-wall which he was building.