I found this game much more entertaining when I would screw up the orders on purpose by dropping all the ingredients outside the taco or giving them uncook meat or making towers. The customer inspects the taco and they're like "WTF?!?!" and the character turns back looks at me and smiles like "What they gonna do about it?"
I suggest a epic upgrade that becomes available to the unit after all other upgrades have been obtained. This way all the extra accumulated exp. is used for some purpose as it would be a fairly expensive upgrade and it would make the game a little more balanced towards the player instead of the opponents having all the awesome units.
 "Papa's Taco Mia!"){kind=icon}
 "My Little Army"){kind=icon}
 "Kingdom Rush"){kind=icon}
 "Sugar, sugar"){kind=icon}
 "Frontier"){kind=icon}
 "Enigmata"){kind=icon}
 "Epic war"){kind=icon}
 "Epic War 5"){kind=icon}
 "Epic Battle Fantasy 3"){kind=icon}