On one of the afternoons we were in Paris, we decided to go outside of the city and check out a fortress. We took the subway all the way to the east and visited the Château de Vincennes, a 14th century fortress. This place used to be a 12th century hunting lodge, but it was expanded to its current size during the reign of Louis XIV.
There are plenty to see here. You enter the massive complex through a narrow gate. Here you are impressed by how thick the walls are. And inside, there are several buildings to explore, but first you buy your ticket from another building inside the grounds. The chapel and the keep are open for exploration. Some of the photos I have above are taken from the keep, which is 52 metres high.
The chapel has eye-catching stained glass, and the building as a whole was completed in 1552. It also houses a photography exhibition, at least during the time we visited.
There are plenty of notable incidents that happened in this chateau; the chateau's Wikipedia entry would provide you more details. In any case, it is an interesting place to check out. It's probably not on every Paris visitor's list, but I must say I enjoyed exploring every corner of it.