We stopped in Linz specifically to go to Mauthausen Memorial. Hard to put into words what it was like, but certainly worth a visit! Most of the site looks like it has hardly been touched in the past 60 or so years. Make sure you watch the video before you leave - goes for about 45 mins, in a few different languages. Found the map a little hard to follow with the audio guide, but just walked around and listened to different entries on the audio guide. A very moving place! I was a little scared off about how to get there. In the end we stayed in Linz, went to the main bus station (Lin Hbf - behind the main train station) and took the bus 360 (you can get 360 or 361). We told the driver where we were going and he dropped us off one stop before Mauthausen town (don't go into Mauthausen town - that's too far to walk!). From where we were dropped off (just passed a sign that points to Mauthausen Memorial up the hill) we then walked up the fill, took us about 15-20 minutes to walk up there, not a bad walk really. The buses are not very regular, on the way back maybe one every hour, so get a timetable when you are there.
2 euro entry which includes audio guide. You pay a deposit for a locker but get it back. We did manage to get a lift back with an English guy to the bus stop - he only stayed a few minutes but got too emotional so left quite early.
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