“The 7th Army, comprising nine divisions, a tank corps and three tank brigades, was located on the Karelian Isthmus. Its objective was to quickly overrun the Finnish defenses on the Karelian Isthmus and conquer Viipuri. From there, the 7th Army was to continue towards Lappeenranta, then turn west towards Lahti, before the final push to the capital Helsinki. The force was later divided into the 7th and 13th Armies.”
That 7th Army was the one that received extremely heavy casualties in their attempts of a breakthrough.
Also, the land concessions before starting this war demanded removal of defensive lines and a military base near Helsinki (the capital). This was just diplomacy as a means to make the war easier.