The Speyer Cathedral is the dominating building in town. It is also called Kaiser- und Mariendom (Cathedral of emperors and Mary). Its size and its importance in history dwarf the surrounding town by far.
Like other major churches in Germany the Speyer Cathedral is called a "Dom" (dome). This has nothing to do with architectural features, although it has a dome over the crossing, and one over the western building, but it is a description for the hirarchically standing in the church.
The building was started in 1030 by the first Salian Emperor Konrad II and consecrated in 1061. It has changed quite a bit since then. But since during the altertions and re-buildings the original romanesque concept has beem conserved it has been awarded with UNESCO world heritage status in 1981.
Today the Speyer Cathedral is the biggest remaining romanesque church in Europe. And it is, like almost every big and old church in Germany, property of the catholic church.