Also, it's a good idea to start by taking a hero with a specialization in Cerberus and using them to clear out Cerberus nests. Firstly, due to their high speed, and secondly, due to their retaliation and three heads, you can at least eliminate quite strong Necropolis units with minimal losses...

But in general, Castle isn't the best choice, I wouldn't risk playing against a human opponent with it. But for offline play, it's quite suitable! You can come up with some tactics...