In brief
Art nouveau is an important layer, but it does not alone explain the entire historical center.
Useful to know
Riga is an important Hanseatic center, the heyday of which in the XIII-XV century is associated with trade. In the 19th century, new neighborhoods formed around the medieval core, including wooden neoclassical buildings and later Jugendstil-style architecture, also known as the Ar Nouveau. [1]
When the city presents itself only through ornate facades, its various layers are easily lost. A more accurate reading compares the medieval structure to the later expansion. This also allows to see a change in the scale of urban spaces, not just in ornamentation.
When viewing, note from which area a photograph is given. A detail of a facade cannot present the device to the entire center on its own. Riga's history is suited to the question of how economic development leaves different, adjacent architectural environments instead of one style obliterating the previous ones altogether.
Sources
For further detail and to check the information: