Learn Spanish (Latin America): El castillo de hielo quedó deslumbrante.

You would use qué tan followed by the adjective: ¿Qué tan deslumbrante quedó el castillo de hielo? This is the standard Latin American way to ask about the degree or extent of a specific quality.

Why is quedó translated as 'turned out' here instead of 'stayed' or 'was left'?

When quedar is followed by an adjective like deslumbrante, it frequently expresses the final result of a process, creation, or transformation. In these contexts, it translates most naturally to 'turned out' or 'ended up' in English.

Why do we say castillo de hielo instead of having a direct adjective for 'ice'?

Spanish frequently uses de plus a noun to describe the material something is made of. Unlike English, which can simply stack nouns together like 'ice castle', Spanish requires this prepositional phrase to link them.

How is the adjective deslumbrante formed?

It comes from the verb deslumbrar, which means 'to dazzle' or 'to blind with light'. Adding the suffix -nte turns a verb into an adjective describing something that performs the action, very similar to the English '-ing' ending.