Being in Marktl and Altötting, you might also consider going to Burghausen which has the longest castle in Europe (> 1km). Wikipedia has a nice article with a few pictures: The article is available in German, Bavarian and English. It is one of the many hidden secrets of South Eastern Germany. ^

BTW: In case you talk German, don't be surprised if you have problems understanding the local slang. Most of my friends needed quite a few weeks to understand Bavarian - and they are still far from understanding everything.