The Kärnäkoski Fortress area is classified as a heritage landscape of provincial value. In summer, grazing sheep keep the landscape open.
Many key plant species characteristic of heritage landscapes thrive in the meadows of Kärnäkoski, including the threatened Lady's Bedstraw (Galium verum), Large Brown Clover (Trifolium spadiceum) and Maiden Pink (Dianthus deltoides). In all, 130 vascular plant species have been identified in the inner courtyard of the Fortress, including several so-called archaeophytes originating in Russia, which are reminders of the Fortress' history. The impressive Dark Mullein (Verbascum nigrum) was used medicinally, as was the rare Small Meadow Rue (Thalictrum simplex) and Ribwort Plantain (Plantago lanceolata).
During renovations at the fortress, the soil seed bank that has accumulated in soil layers over the years has included Tower Mustard (Arabis glabra) and Henbane (Hyoscyamus niger), the latter being a highly poisonous plant used as painkiller and known as early as the classical period. Hoary Alison (Berteroa incana), known in folklore as "the Cossack's footstep", is evidence of the site's military history.