Interesting building from La Citadelle de Quebec

The Citadelle de Quebec is a military base on the highest point of Quebec. French, English and now Canadian forces have occupied this land. The building is on the grounds of the fort.

This building has no direct military use. I ask you to tell me what use it has from the design and clues:
1. a book we both read about a year ago
2. the book was about an English man.
3. the St. Lawrence River is in view of this building.
4. the metal "ball" is a key