Simplify the following text for a Grade 5 native English-speaking person.
Original: It was a politically important seat of the Prince-elector of Mainz ( see : Archbishopric of Mainz ) under the Holy Roman Empire , and previously was a Roman fort city which commanded the west bank of the Rhine and formed part of the northernmost frontier of the Roman Empire . Mainz is located on the river Rhine across from Wiesbaden , in the western part of the Frankfurt Rhine-Main Region ; in the modern age , Frankfurt shares much of its regional importance .
Simplified: Mainz is located on the left side of the river Rhine , on the right side is the city Wiesbaden .