Ammianus Marcellinus Res Gestae 21.10.3

[3] The closely united summits of the lofty mountain ranges Haemus and Rhodope, of which the one rises immediately from the banks of the Danube and the other, from those of the Axius, ¹ on our side, end with swelling hills in a narrow pass, and separate Illyricum and Thrace. On the one side they are near to the midlands of Dacia and to Serdica, ² on the other they look down upon Thrace and Philippop- olis, ³ great and famous cities; and as if nature had fore-knowledge that the surrounding nations must come under the sway of Rome, the pass was purposely so fashioned that in former times it opened obscurely between hills lying close together, but afterwards, when our power rose to greatness and splendour, it was opened even for the passage of carts; and yet it could sometimes be so closed as to check the attempts of great leaders and mighty peoples.