Explanations

  1. an ancient castle: a singular countable noun needs an article; an before a vowel sound.
  2. went for a walk: fixed expression.
  3. go to the beach: go to + place.
  4. museum to learn (purpose) or and learn (two actions): both are accepted.
  5. when I get back home: present simple after when for the future. am and come are also accepted.