You should definitely make time for this during your trip to Brussels. For 6 euro, you get a tour of the brewery and an education on the brewing and history of lambic beer. The building itself is historic and the beer could be likened more to wine than a commercial/large scale brew. The fourth generation of brewers is now running Cantillon, I hope they continue the tradition! Oh, and after the tour, you get to try both the Lambic and a fruit version of the beer, lambic mixed with either strawberries, cherries, or rasberries. It's really quite a unique experience for your trip to Belgium. (TIP: We had our stroller, but with the stairs it's quite impossible to take the stroller around the brewery, so they owners watch it for us as we walked around with the baby. So my Tip is: go when it's not nap time since you'll have to carry your baby. Also not really handicap accessible, except where you buy/sample the beer, so if you want to experience Lambic and see some limited parts of the brewery, I'd say it's still worth it.)
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