 In 1950/51 he discovered the foundations of the Church of the Virgin Mary in the castle grounds which, dating from the second half of the 9th century, was the oldest church in the castle.Borkovský also carried out research into the Convent of St. Agnes, Convent of St Anna and the demolished Bethlehem Chapel in the Old Town, which were important to building an understanding of the medieval development of Prague.He led the investigation of the Levý Hradec until 1954 and was also involved in research into the monastery of St. Jiří.