I answered "Eu queria andar ao castelo" and lost a heart. I think I need a little lesson in the difference between "a+o=ao" and "até o". The "até a/o" construction does seem to be preferred sometimes and a little hint about when would be good.
"Até o" shows your last destination. For instance, someone is going for a walk and another one asks her "até aonde você irá caminhar?", then he answers: "Até o lago" (Up to the lake) - Here is one more example of the usage of the word até.
Well, we don't use "andar a" but "andar até" or "ir a/até" when you want to express where you are going. We say "vou andar no castelo" meaning you'll walk around in the castle, but not "vou andar ao castelo"