You'll probably spot St. Polten Cathedral from a distance before you actually find this important religious building. Pop inside if the doors are open or just admire the facade from the street.
Located in St. Pölten, this principal church is well worth a look. But you'll find plenty more to see in the area. Domplatz, Sankt Polten City Hall, and Kamptal Gardens aren't too far from St. Polten Cathedral.