Ita maximo terrore perculsi Fidenates, prius paene quam Romulus equitesque qui cum eo erant circumagerent equos, terga verterunt [tergum vertere : fugere] atque oppidum repetebant. Non tamen a persequentibus se eripuere [eripuere = -erunt]: priusquam fores portarum opponerentur, Romani velut uno agmine in oppidum irruperunt.
The second sentence seems to be saying that despite fleeing their pursuers the Fidenates were unable to block the gates to their city before it was invaded by the Romans.
Now I find Orberg's punctuation (if my reading of the Latin is correct) somewhat strange. Because it seems to me that the sentence should be punctuated as follows:
Non, tamen a persequentibus se eripuere, priusquam fores portarum opponerentur, Romani velut uno agmine in oppidum irruperunt.
1) Why is he using a colon after eripuere?
2) Am I right in saying that the word Non governs opponerentur?
I am sometimes at a loss to understand Orberg's punctuation.