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Sidirokastro -LRB- Σιδηροκαστρο Bulgarian and Macedonian Slavic  : Валовища  /  Валовишта Valovista ; Demirhisar -RRB- is a town and a former municipality in the Serres regional unit , Greece .. Serres. Serres ( regional unit ). Bulgarian. Bulgarian language. Macedonian Slavic. Macedonian Slavic. Serres regional unit. Serres ( regional unit ). Greece. Greece. Since the 2011 local government reform it is part of the municipality Sintiki , of which it is the seat and a municipal unit .. Sintiki. Sintiki. It is built near the fertile valley of the river Strymonas , on the bank of the Krousovitis River .. river Strymonas. Struma River. Sidirokastro is situated on the European route E79 and the main road from northern Greece -LRB- Thessaloniki -RRB- to Bulgaria .. Greece. Greece. European route E79. European route E79. Thessaloniki. Thessaloniki. Bulgaria. Bulgaria. It has interesting sights , such as the medieval stone castle , Byzantine ruins , natural spas and beautiful nature .. Byzantine. Byzantine