Västerås Castle stands at the southern edge of the old town, a compact round tower flanked by later Renaissance wings that transformed a medieval fortress into an administrative palace. The original tower dates to the 13th century; the characteristic red-and-white wings were added in the 16th century under Gustav Vasa. Today the castle houses the county administrative board.
The exterior is freely accessible and worth a walk around — the building sits above the Svartån river, with good views from the bridge below. The castle gardens are open to the public in summer. Check locally for guided tours of the interior, which are available on certain dates.