"Yes, they are castles."
Translation:Ano, to jsou hrady.
I am sorry to say that Ty jsou hrady. doesn't sound very natural in Czech.
In this case, you use to: 'to' refers to things or persons which are not defined – or not yet defined – by particular nouns. It often occurs in questions, and in defining statements, which may be answers to such questions. In such contexts English often uses ‘it’, which is less emphatic than ‘that’...' [...] 'Singular 'to' is also used when identifying plural things or persons...' (from Czech: An Essential Grammar by James Naughton, 2005.)
I wish that someone would post, or at least lead me to a site so that I could fully understand and have an easy to use and remember table for: ten, ta, to, ti, ty (and whatever else) are related to singular and plural Masculine animate, Masculine inanimate, Feminine animate (is there Feminine animate?), and Neuter animate and inanimate. Grazie mille.