Learn German: In der Hauptstadt gab es ein Schloss. - In the capital, there was a castle.

The basic phrase for 'there was' is es gab. However, because this sentence starts with the location In der Hauptstadt (In the capital), the verb gab must take the strict second position in the sentence. This pushes es directly after the verb.

Would es gab change if there were multiple castles?

No. Just like es gibt translates to both 'there is' and 'there are', the past tense form es gab works for both singular ('there was') and plural ('there were'). It stays exactly the same even if you are talking about many things.

Why does the sentence use der Hauptstadt if Stadt is a feminine noun?

The preposition in is a two-way preposition. Because the sentence describes a fixed location (where the castle is) rather than movement towards a destination, we must use the dative case. In the dative case, the feminine die changes to der.

What case is ein Schloss in this sentence?

It is in the accusative case. The phrase es gibt (and its past tense es gab) always takes the accusative case for the thing that exists. Because Schloss is a neuter noun (das Schloss), its accusative word ein looks identical to its basic subject form.