The building the shop is in is a 2-story shop/housing complex. There are typically apartments on the second floor and in back behind boutique-style shops like Lavender. The front door offers access to the apartments as well as the door to the shop. The roof is moderately pitched and covered with red terra cotta tiles. Stairs offer access to the second floor in the hall just past the door to Lavender as well as right next to the storefront window.
From their vantage point on the roof, there doesn't actually appear to be any easy access from the rear (fire codes not being much of a thing in Korvosa). There are no windows in that direction, for example. All the windows point out the front of the building - including shops on the ground floor and apartments above (generally shuttered).
The view in the front window shows Vendra greeting customers and otherwise directing them about while two young women clerks handle the transactions with customers from behind a counter. There are two more husky but well-dressed guards in the main shop.
I've put a map together (with a long queue of commoner tokens) to indicate the extent of people milling about. I've placed Cassandra and Amira roughly on the roof. And I've exposed the shop's interior, though if Cassandra or Amira wanted to be directly above that space, we could just put a border on them to indicate they're on the roof.