Short answers, pulled from the story.
The Karelian Isthmus is situated between the Gulf of Finland and Lake Ladoga in northwestern Russia, north of the River Neva. Its area is about 15,000 square kilometers, spanning the city of Saint Petersburg and Leningrad Oblast.
Following the Winter War, approximately 422 thousand Karelians were evacuated from the ceded areas to other parts of Finland. After the Continuation War ended in 1944, people from other parts of the Soviet Union, mostly Russians, were settled in their place. Around 1948, the government renamed the vast majority of Finnish place names to invented Russian ones.
The Mannerheim Line was a Finnish defensive fortification that crossed the Karelian Isthmus during the Winter War of 1939-1940. Soviet forces did not manage to penetrate it until February 1940, after suffering disproportionately heavy losses in the early weeks of the conflict.
The highest point of the Karelian Isthmus lies on the Lembolovo Heights moraine at about 205 meters (670 feet). The terrain was shaped by the Weichselian glaciation, which left behind moraines, massive granite boulders, and numerous lakes.
Finland ceded the Karelian Isthmus following the Winter War under the Peace of Moscow, signed on the 12th of March 1940. Finland regained the territory briefly during the Continuation War but ceded it again under the Moscow Armistice of the 19th of September 1944, a border confirmed by the Peace of Paris in 1947.
Notable people from the isthmus include Martti Ahtisaari, Finnish president and recipient of the 2008 Nobel Peace Prize, and Artturi Ilmari Virtanen, recipient of the 1945 Nobel Prize in Chemistry. The poet Edith Södergran and soldier Lauri Törni, born in Viipuri, also came from the isthmus during its Finnish period.