Even though only 1 of this item can be held in the inventory, the player can pick up, as many keys as he or she wants, getting points for each one, even thought you can only carry one.
Beyond Dark Castle Version
In Beyond Dark Castle they added the ability to hold more then one key, being able to hold up to 50 keys.
This system is kept in Return to Dark Castle
The appearance of the Key has stayed pretty much the same between all versions of Dark Castle.