Vienna began as a Roman legionary camp called Vindobona, built around the end of the first century on a bluff above the Danube and a small tributary. The camp held a legion, first the Thirteenth and later the Tenth, and it was a large rectangle surrounded by a wall and ditch, with gates on each side. A civilian settlement grew outside it. The Roman emperor Marcus Aurelius died in this region in 180, and one tradition says he died at Vindobona, though the evidence is not certain.
The camp's outline survives in the street plan of the old city. If you look at a map, you can see how the lanes of the Innere Stadt follow the line of the Roman wall, curving where the corners once stood. Hoher Markt, a square in the middle of the old town, has an underground museum where you can walk through Roman house foundations, and a short walk away, on Michaelerplatz, the excavated remains of Roman buildings are visible in front of the Hofburg. The Danube Limes, which includes Vindobona, was listed by UNESCO in 2021.
Vienna of course has many other layers: the Habsburg palaces, the Ringstrasse, the coffeehouses, the music of Mozart and Beethoven and Strauss. It is a big city, with about one and a half million people, and everything a rider needs is available. Fuel is easy to find on the outskirts, and the centre is compact enough that you will want to park and walk. Parking in the inner districts is regulated by short-term zones, so look for the signs and avoid fines.
To get a sense of the city from above, ride up to the Kahlenberg, a hill on the northern edge of town that gives a fine view over the roofs and the river. The road climbs through vineyards, and on the way you pass wine taverns, a Viennese tradition. In the evening a glass of new wine and a plate of cold meat and bread is a good end to a day. Be careful with tram tracks, which can be slippery when wet, and with cyclists, who are numerous.