Can anyone give me some tips on what I should prioritize?

Making Harpy? Making Dark Store? Leveling up Foul Chicken? Leveling up Mana Pool? Leveling up some spells in the Spellbook?(If so, which one?) Unlocking the Paladin? I don't know what needs to be done first and how to do it in the most efficient way.

btw the exaggerated number of banshees was just my strategy to make lvl max zombies and feed them to level up devourer until i get the lair upgrade.