Complex sentence: At that time Siedlce emerged as one of the most important cultural centers of the nation, the "Ogiński Palace" was visited by several notable artists and writers, such as Franciszek Karpiński, and Julian Ursyn Niemcewicz.
Split into simpler sentences: At that time Siedlce emerged as one of the most important cultural centers of the nation. The "Ogiński Palace" was visited by several notable artists and writers, such as Franciszek Karpiński, and Julian Ursyn Niemcewicz.