Complex sentence: They are populated by a lengthy cast of characters, and set firmly in Bellman's time and place, eighteenth century Stockholm, but are simultaneously decorated, for romantic or humorous effect, in Rococo style.
Split into simpler sentences: They are populated by a lengthy cast of characters, and set firmly in Bellman's time and place, eighteenth century Stockholm. But simultaneously, they are decorated, for romantic or humorous effect, in Rococo style.