From St. Stephen’s main entrance you can admire its impressive west facade with the giant’s door and heathen towers just as people did in the Middle Ages. It hasn’t really changed much in 750 years and is the oldest remaining part of the cathedral. The original church of St. Stephen is consecrated in 1147 but is rebuilt entirely after a terrible fire devastates half of Vienna in 1258. During the 5 years of renovations the western wall takes on its current appearance.
The new church is far too big for the city at that time; in the mid 13th century Vienna is an up-and-coming capital, but hardly a metropolis. And above all, still part of the diocese in Passau. The sheer size of the new church, however, could be an indication of the Prince-Bishop in Passau’s intention to raise St. Stephen’s to cathedral status. But this won’t happen for another 200 years.
The consecration of the first church of St. Stephen in 1147 does not mean that the building is complete at this time. There are no documents or records to support this. It’s not unusual for medieval churches to be consecrated as soon as services can be held in them, even if they’re half finished. Towers and extra features are simply added on later.