BIRKENFELD, a market town in West Germany, 2 5m. E.S.E. of Trier, capital and seat of government of the province of Birkenfeld, in the Land of Oldenburg. Pop. 3,194. Above the town lie the ruins of a castle (r4th century), once the resi dence of the counts palatine of Zweibrucken. The province (for merly principality) of Birkenfeld is hilly and well forested. Pop. It is entirely an enclave in Prussian territory. Ironstone and roofing slates are quarried and agate-polishing is practised at Idar.