The Hero's Path

And can I get one more piece of advice? I've decided to replay the game from the beginning. I completed the peasant quest, bought 4 Paladins, and I have around 6,000 gold in the bank, which is accumulating. So far, I'm clearing the area before the jumping demons and unicorns relatively easily, although Centaurs have already killed my archers. Should I buy more Paladins before facing the unicorns and demons, or should I buy some other units? I started playing as the Knight.

Added 1 minute ago
And one more thing. Should I immediately buy the Alchemy skill in the first Castle? I haven't used this feature yet, but after reading the comments, I thought it might be quite useful.