There are secret crafting recipes; certain combinations of body parts will unlock special recipes that grant extra stats to the unit. They are basically always better than random parts cobbled together, so you should use them as much as possible.

If you place the body parts in the crafting table, then press save, it will tell you if it's a rare recipe or not without you having to actually expend the mana or parts to find out. I wouldn't bother actually saving anything, since the rare recipes are saved to the rare recipe list anyway, & you really don't want to make anything else unless it's the very start of the game when you should be churning out as many minions as you can from whatever you have & setting them to gather in order to get your economy going, or your base is being attacked & you desperately need some cannon fodder.

Their are 4 tiers of recipe:
Legendary > Epic > Rare > Uncommon

To start with you will have to set all of your dudes to gather resources (the hand icon), switching them back to defence (the castle icon) when you are about to be attacked (this happens at night). Once you have filled all your minion slots you will be better off putting the slower half of your minions on defence while the faster half (or minions with wings or other terrain crossing abilities like amphibious or lava walking) are all set to gather resources. Once you have built your first titan you should be able to rely on them for defence while everything else gathers resources. When you progress to unlocking additional titans set them all to defence. The reason for this is that bosses will just instantly delete everything else anyway, so there's just no point, & it's a waste of resources. Enemy lairs should be dealt with using lightning & meteor strikes.