In theByzantine Empire, akleisoura (Greek:κλεισούρα, "enclosure, defile") was a term traditionally applied to a fortifiedmountain pass and the military district protecting it.^[1] By the late 7th century, it came to be applied to more extensive frontier districts, distinct from the largerthemata, chiefly along the Empire's eastern border with theCaliphate along the line of theTaurus-Anti-Taurus mountains (in theWest, onlyStrymon was in its early days termed akleisoura).^[1] Akleisoura orkleisourarchia was an autonomous command, under akleisourarches (Greek: κλεισουράρχης). Eventually, mostkleisourai were raised to fullthemata, and the term fell out of use after the 10th century (in late Byzantine times,droungos had a similar meaning). ItsIslamic counterpart inCilicia andMesopotamia was theal-thughūr.^[1]

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