For anyone who ever makes it to Barcelona, the Museu Nacional d'Art de Catalunya (MNAC), the art Museum in the Palau Nacional on Montjüic is great. The only other thing I've seen there by way of museums which even begins to compare is the Picasso Museum (and, of course, there is Sagrada Familia, not exactly a museum, but, hey...). The museum at the Palau Nacional is in the midst of reopening section by section after years of restoration. So far  they mainly have the Romanesque collection on display, and it's enough in its own right to be a great museum. (For those of you who may not have slogged through art history classes, Romanesque is the style of the early middle ages, before the style known as Gothic.)
Fundamentally, it's like getting to visit 25 Romanesque churches in a single day. Many years ago, someone noticed that the Romanesque churches of northern Catalunya were falling into disuse and disrepair and had the idea of salvaging the art (especially the frescoes) while there was still something to salvage. These are now in the museum; they've done a great job of building wood and canvas structures in the exact shape of the apses, etc., they took the work from, so as to pretty much replicate the way the art stood in the original churches, right down to painted window cavities, and the like. A lot of really first rate stuff, certainly the best collection of Romanesque work in one place that I've ever seen, and according to Marc Ignasi (expert on the Romanesque, but possibly biased in favor of Catalunya) it's the best there is.[Addendum 2002: Marc and I have had some further back and forth on this, and while he may have bias in favor of Catalunya, in this case he is certainly correct. Because Romanesque art is virtually entirely religious art, it was virtually all produced to adorn churches and monasteries. There is certainly no other museum where so much of it has been gathered together, not to mention the fact that Catalunya was certainly one of the most notable artistic regions in that era.
By the way, the Gothic collection and the temporary exhibition spaces are now up and going, and this museum is even more impressive today than when I visited in '96.]
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