Learn Italian: Compreremmo un castello grande nel sud. - We would buy a big castle in the South.

For regular verbs ending in -are like comprare, the a changes to an e when forming the conditional tense. The base becomes comprer-, and then you add the 'we' ending -emmo to get compreremmo.

Where is the word for 'we' in this sentence?

It is built into the verb! The -emmo ending on compreremmo specifically means 'we would'. In Italian, you usually leave out the subject pronoun (noi) because the verb ending makes it completely clear who is doing the action.

Is there a difference between un castello grande and un grande castello?

Yes, a subtle one. Putting grande after the noun (un castello grande) focuses on its physical size: a bulky, large castle. Putting it before the noun (un grande castello) can mean it is 'grand', magnificent, or important, not just physically big.

Why use nel instead of just in for 'in the South'?