Learn Arabic (Modern Standard): كانت القلعة ضخمة.

The word for castle, القلعة, is a feminine noun because it ends in a taa marbuuta (ة). Therefore, we have to use the feminine form of the verb 'to be', which is كانت (literally 'she was' or feminine 'it was').

If I listen closely, what vowel ending does the word ضخمة (huge) have in this sentence?

It takes an 'an' ending sound, making it pronounced as ضخمةً (ḍakhmatan). This is because in sentences using كان (to be), the describing word (the predicate) must be in the accusative case. In Arabic, we show this on an undefined adjective with a double fatha.

Why does the verb كانت come first?

Arabic sentences usually start with the verb. So instead of 'The castle was huge,' the literal word order is 'Was the castle huge.' Starting with the noun (القلعة كانت ضخمة) is also possible, but starting with the verb is the standard, most natural way to say it.

Why does the adjective ضخمة also end with a ة?

Adjectives in Arabic must match the gender of the noun they describe. Since القلعة is feminine, the adjective for 'huge' also needs the feminine marker ة, changing the masculine ضخم into the feminine ضخمة.