Learn Spanish (Spain): El viaje al castillo fue increíble. - The trip to the castle was incredible.

The verbs ser and ir share the exact same forms in the Pretérito Indefinido (simple past tense). Here, fue comes from ser and means "was". You can tell by the context: "the trip went incredible" doesn't make as much sense as "the trip was incredible".

Why do we use fue (from ser) to say how the trip "was", instead of a past tense form of estar?

While estar is often used for temporary states, ser is used to describe the overall qualities and characteristics of an event. When you give your opinion on how an event (like a trip, a party, or a class) was, you always use ser.

Why does the sentence say al castillo instead of a el castillo?

In Spanish, whenever the preposition a (to) is followed immediately by the masculine word for "the", which is el, they must join together to form the contraction al. You will never see a el written as two separate words.

The word viaje is masculine. Why doesn't increíble change to match its gender?

Adjectives that end in the letter -e are gender-neutral in Spanish. Words like increíble (incredible), grande (big), and inteligente (smart) use the exact same form for both masculine and feminine nouns.