Siesel, also called European Ground Squirrel (Spermophilus citellus) in Vienna.
Siesels live in the eastern European Steppe grasslands. They are endangered because of habitat destruction.
Fortunately in some places they are still thriving. In the Vienna metropolitan area live an estimated 16.000 Siesels, mainly in protected area's.
IUCN Red List: Endangered
Siesels are easy to observe in the right places, but DO NOT FEED THEM! They are wild and can find their own food. You will disturb their natural behavior while feeding them.
It's also not necessary; you can see them foraging closely if you stay quiet and just observe.