 The Finns had built 221 strong @-@ points along the Karelian Isthmus , mostly in the early 1920s . Many were extended in the late 1930s . Despite these defensive preparations , even the most fortified section of the Mannerheim Line had only one reinforced concrete bunker per kilometre . Overall , the line was weaker than similar lines in mainland Europe . According to the Finns , the real strength of the line were the " stubborn defenders with a lot of <unk> " – a Finnish idiom roughly translated as " guts , fighting spirit . " 
