This quiz should probably have a yellow box. Currently it's way to easy to just go through the capitals and not even look at the clues. Also, if I got a question wrong, the answer would often pop up as the correct answer for a later question.
Budapest also has yellow trams, and I've taken photos of them as a tourist. I've never been to Lisbon so wasn't aware of the presumably more famous ones there.
Stockholm has also been called the "Venice of the North" (as have several other non-capital cities) and probably has that many islands if you include the greater Stockholm region. It may not have enough bridges, though, to be an alternative answer.
Interestingly, that type of jelly donut is called a "Berliner" in most of Germany, but... not in Berlin itself! In Berlin, it's called a "Pfannkucken" (pancake).
I cannot for the life of me remember why I knew Prague?! I don't remember any details of the defenestrations. I don't know where I would have heard that fact, but the moment I read the clue I knew it was Prague. Brains are weird.
Nice quiz though, thanks!