I only saw Luxembourg on a daytrip from Brussels, but it was a very nice experience. On the one hand, Luxembourg is a European capital with lots of banks and office buildings in a multi-cultural and multi-lingual environment, on the other hand its old town is very well preserved and interesting, especially the cathedral and all the fortifications. Luxembourg was once the most heavily fortified place in Europe, and it shows. Not all remains, but what you can still see gives you a great impression of military architecture, of forts and castles, ramparts, walls, cannon, etc. A worthwhile trip not only for history buffs.