Herodotus tells us that the Russian Cimmerians built castles or forts,[501] a custom which is found among the early iron age or Hallstatt inhabitants of the mountain zone,[502] and reached this country somewhat later in the form of Hill-top camps. Their distribution has not yet been well worked out, but their date is Hallstatt or sometimes later, and the available evidence from their distribution in time and space suggests that they were the work of different branches of the Kimri. A large number of the Kimri, perhaps the greater part, remained in the Koban region until the seventh century, when they were displaced by incoming Scythian hordes, who appear to have been of mixed Iranian and Mongol origin; then they overran Asia Minor as far as Sardis.[503] But many of these Kimri left the steppe almost immediately after they had developed their iron swords and settled in Thrace; later they moved up the Danube valley as far, at least, as its junction with the Save. It was not long before the bulk of them moved southwards, probably down the Vardar valley, and about 1000 /B.C./ began the Dorian invasion of Greece. These introduced into that country iron swords and a P tongue, which, owing to their having mingled with Iranian neighbours in the steppe, retained marked affinities with that group of languages, especially in connection with weapons and other warlike materials. The remainder divided, the larger group pushing up the Danube valley towards Ulm and Sigmaringen where they adopted the Celtic speech of their subjects, but labialising the Qs. The smaller group made themselves masters of North Serbia, Bosnia, and Croatia, and like their fellows adopted the language of the country, which was allied to Latin, but with the usual changes.