Technological holes

I do not know exactly what it is and what it for. Probably this is a cable channels, as well as pipes for heating the turret in the winter. However this is all my idle speculation ...

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Land forts and fortress:
Bip Castle Gatchina Ivangorod Izborsk Kexholm Kirillov Monastery Koporye Novgorod Pechorskiy Monastery Peter&Paul Fortress Porkhov Pskov Schlisselburg Staraya Ladoga Tikhvin Vyborg Hameenlinna Hamina Kastelholm Kymenlinna Lappaenranta Raseborg Castle Savonlinna Tavetti Turku Visby Fredrikstadt Fredriksten Hegra Fort Hoytorp Fort Arensburg Narva Tallinn Antipatris Caesarea Jerusalem Latrun Fort Masada
Sea forts and fortresses:
Alexander Fort Ino Fort Krasnaya Gorka Fort Kronstadt: Kotlin isl. Kronstadt: North Forts Kronstadt: South Forts Trongsund Hanko Svartholm Sveaborg Marstrand Siaro Fort Vaxholm Oscarsborg
Artillery batteries and individual guns:
Coastal Artillery Hemso Fort
Fortified areas and defensive lines:
Karelian Fortified Area (KaUR) KrUR Leningrad Mannerheim Line Nevsky Bridgehead VT Line Harparskog Line Salpa Line Gothland

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