I never used the subway when I was in Sofia, primarily because the central attractions are all within walking distance of one another and it seemed that the subway system lead out of the centre. Nevertheless, Serdika station (named after the original name of Sofia) is the primarily means of crossing streets under the traffic of the city centre. Serdika is a large underground complex (like many cities, Sofia has combined its subway stop with a sort of underground shopping mall). In addition to the children playing classical music, you can find all sorts of archeological monuments dug up, presumably, in the process of constructing the subway.
there are some remains from Serdika/Sredets - the ancient Sofia,
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the exite of subway station Serdica, in the heart of Sofia also the statue of sofia the statue of sofia is gold plated.