- Life related
- Resource related
- All Tower stats or cost modifier
- Specific tower stats or cost modifier
- wave count requirement - player need to survive at least how many wave before the item become available
- perk point requirement - how many item player need to purchase and unlock before the item become available. Note that purchase an item gives player 1 perk point
- pre-req perk - what are the perk/item needed to be unlocked before the item become available. You can have as many of this. Even a perk that require unlocking every other item in the list!