Kalinigrad and Central Poland are separated by maze of lakes, forrest and marshes 100 km in-depth...Not exactly a tank country - Masuria had always been considered a major strategic obstacle..
Same goes for Polish-Byelorussia border - large forrests, rolling hills...
Each Russian staff officer worth his salt knows details of this terrain from studying history - 1793, 1809, 1831, 1914, 1920, 1941, 1944... And impassable has different definition in XXI century then in 1812... But still...it is a terrain well suited for defense...
Situation is somewhat similar here in Lithuania. Unfortunately what I learned studying our post-WW2 resistance, it's not as obstacle as it was back then. Melioration is probably #1. Marshes are anti-tank canals, with bridges supporting tractors and trucks. Then greatly improved road network. Especially access roads for farms, forest service and so on. Impenetrable marshes and forests of 1940s where some cells held out for a decade are now zig-zagged with roads that support trucks overloaded with timber. Farms have fascinating access roads too. And Polish farms seem to have even better access roads, at least what I saw while cycling down there.
Sure, it's not flatland where you can roll by tanks anywhere. But so was Ukraine in february when fields were sinkholes. And it's nowhere close to proper mountains. Or marshes of 100 years ago.
I have read in Northern Crusades [1] that in XII/XIII century forrests separating Prussia from Lithuania/Mazovia had been truly impassable (except for raiding parties - raizes) and the only campaining had been possible along large rivers. Lituanians had it easier because they had been bringing siedge equipment and canons downstream. So there is some change although meassured in centuries :)
Yes. But the huuuuge forests of XII century were converted into arable land in Commonwealth times. Export timber, then grow grain in place of the old forest and export it... And then unexpected partition happens and you can't hide in the forest anymore :/
