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caistealcastlemasculine noun
the
an caisteal
after a preposition
a' chaisteal
of the
a' chaisteil
plural
caistealan
the (plural)
na caistealan

Also the chess rook, the castle.

Every form but the headword, the genitive and the plural is worked out by the spelling rules in the grammar guide, so what you see here is what the guide teaches. Awaiting a fluent speaker’s review.