Within Old Town, driving is not permitted from 11 AM to 6 AM every day.
This means that cars can only drive right up to the hotel between 6 AM and 11 AM. During the rest of the day, Munkbron or Slottsbacken are the closest places for drop-offs or pick-ups by car or taxi, or Riddarholmen where there's parking spaces.
For mor information on the traffic situation in Old Town, see here.
The image to the left is an adapted screenshot from the same link. The link shows an interactive map with added information in Swedish.
Blue lines: traffic not permitted 11-6 AM.
Purple lines: traffic not permitted 11-6 AM, transportation of persons with limited mobility excluded.