Kaluzhskiye Zaseki Nature Reserve (Russian: Государственный природный заповедник «Калужские засеки») (or Kaluzhskie Zaseki for short) is a Russian zapovednik (strict ecological reserve). Zaseki in Russian means abatises, field fortifications made of felled trees. In medieval times, the Kaluga region was part of the defensive line between the Russian forests to the north and potential invaders from the south. The reserve has been protected in some form for a thousand years, and remains old-growth forest. The reserve is situated in the Ulyanovsky District, Kaluga Oblast. It was formally established in 1992 and has an area of 18,533 hectares (71.56 sq mi).