Final answer:
The sentence in question is a simple sentence because it contains a single subject and predicate and expresses a complete thought without multiple independent or dependent clauses.
Step-by-step explanation:
The sentence "The prince and princess grew up in a large castle with a moat" is a simple sentence. A simple sentence typically contains a single subject and predicate, and it expresses a complete thought. In this sentence, "The prince and princess" serve as the compound subject, "grew up" is the predicate, and "in a large castle with a moat" is the object of the preposition, providing additional information about the location where they grew up. This sentence does not contain multiple independent clauses (like a compound sentence) or dependent clauses (like a complex sentence), so it is categorized as simple.