I discovered this tour by accident: Before my trip to the Netherlands, a friend recommended free walking tours (I have never done them before), and the friend I was travelling with was curious about the hidden courtyards in Leiden.
So we made a quick decision to take a chance and try this tour the same day in the evening, and we didn't regret it.
The guide explained everything, had profound knowledge about the city and its history, and wasn't afraid of any stupid questions no matter how hard I tried.
Also he made sure that we always felt safe in the hustle and bustle of Leiden's bike and car lanes.