Learn Dutch: De vrouw vraagt of wij morgen naar het kasteel gaan. - The woman asks if we will go to the castle tomorrow.

Because the second part of the sentence is an indirect question introduced by of (if or whether). Words like of start a subordinate clause, which means the working verb (gaan) is sent all the way to the end of the clause.

Can I use als instead of of to mean 'if' in this sentence?

No, and this is a very common mix-up! In Dutch, use of when 'if' means 'whether' (like in this yes/no question). You only use als when 'if' sets a condition, such as 'If it rains, we will stay home.' Since the woman is asking whether you are going, you must use of.

The English translation says 'will go', but gaan is just the present tense. Where is the word for 'will'?

Dutch frequently uses the simple present tense to talk about the future. As long as the sentence has a time word that makes the future context clear—in this case, morgen (tomorrow)—you do not need to add a future verb like zullen (will).

Why do we use naar for 'to' in this sentence?

Whenever you are talking about physical movement or traveling toward a destination (like a castle, a city, or an event), Dutch uses the preposition naar.