Bad Blankenburg is a spa town in the district of Saalfeld-Rudolstadt, in Thuringia, Germany. It is situated 6 km southwest of Rudolstadt, and 37 km southeast of Erfurt. It is most famous for being the location of the first kindergarten of Friedrich Wilhelm August Fröbel, in 1837. To the north of it, on an eminence, rise the fine ruins of the castle of Greifenstein, built by the German king Henry I, and from 1275 to 1583 the seat of a cadet branch of the counts of Schwarzburg.