Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Castŭlo, ōnis, m. (Liv.), and f. (Sil.), = Καστούλων, a town in Hispania Tarraconensis, near the borders of Baetica, now the village Cazlona, Liv. 24, 41, 7; 28, 19, 2 sq.; Sil. 3, 99; 3, 391; Plin. 3, 2, 3, § 17.
Hence,