The most important work created in collaboration with Gyldén was the headquarters of Hackman & Co (1909) at Piispankatu 14 in Viipuri. The building represents the compositional and formal variations of Jugenstil, Art Nouveau or National Romanticism. The main elevation pediments, openings, window frames, portals, and the granite facades are quite typical of National Romanticism, which at that time was also the prevailing architectural style in Finland for key public buildings, most notably in the works of architects Gesellius, Lindgren, and Saarinen (e.g. Finnish Pavilion at Exposition Universelle (1900), Paris, and National Museum of Finland, Helsinki (1902–1904).) The interiors of the building were among the most stylish and rich in the city. The inside of the house survived harmful transformations during the Soviet period so the interior decorations were greatly damaged. Between 1910 and 1930 he designed a large number of public buildings and houses, most of which are situated in Viipuri. Among these are several office buildings in Käkisalmi, Imatra and other Finnish towns. The early works in Viipuri could be described as traditional town architecture with a classicistic influence. The most interesting examples from this period include the reconstruction of the Union Bank (originally built in 1900), the Karjala offices (1929), the store and residence of V. Dippel (1921) and the restoration of the famous medieval Round Tower in the middle of the old city centre. Ullberg converted the massive old building into a restaurant and cafe. The most important period in Ullberg's architect career is connected with the "White Era" of Functionalist architecture. However, some of his important works can be considered as marking a transition from Nordic Classicism to Functionalism during the 1920s and early 1930s. Ullberg found his own unique conception and interpretation of functionalistic architecture. The main aspect of his philosophy is the connection between the functional and the classical. In his early works Ullberg used as his basis Nordic Classicism, emphasizing reductive common elements from Classicism. The most important works of the era became Ullberg's reply to the serious social needs of the city. Among these works are the Viipuri Art Museum and Drawing School (1930), the Viipuri Panttilaitos Oy Building (1931), the Viipuri Provincial Archives (1933), and the Viipuri Maternity Hospital (1937).