How do you get the last key?

You need to get both the rareware and nintendo coin here is how you get them both:

1.nintendo coin:Beat the donkey kong game in the factory level twice.on the first time you complete the game you get a golden banana.Next time DK will be holding the nintendo coin.

2.Rareware coin:First get 15 banana medels(Earned by getting you character 75 banana of his colour) then go to any cranky lab.Cranky will as you to play a game of Jetpac.Get a score of 5,000 or more and the Rareware coin will start to appear in random places make sure you grab it before you die!

Now you know how to get the last key!Also in the room are 2 banana fairies!

LATER :D

Like Metel mario said, you need the Nintendo Coin AND the Rareware Coin.

To get the Nintendo Coin, go back to the old Donkey Kong Arcade Game in Frantic Factory. You probably already beat it the first time to get the Gold Banana, so do it again to get this Coin.

To get the Rareware Coin, you need 15 Banana Medals which are earned by getting 75 Bananas for a character in a level. You probably already have these, so go to Cranky's Lab and play the old Jetpac Game - if you get more than 5000 points, you'll find this Coin.

Now you'll be able to enter that room and get the last key!