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Once again, you take the role of Mr. Do in his second adventure. Mr. Do has lost his power ball throwing ability, but in it's place, he has received a magic hammer. Arguably a weaker weapon, the hammer is, none the less, Mr. Do's only means of offense. Mr. Do's primary goal is survival, as there are a number of ways in which he can clear the current stage and advance to the next. He can either pop out all of the cherry blocks from the floors with his hammer, he can kill every monster in the stage, he can collect every letter in the word "EXTRA", or he can collect the ultra-rare free game diamond. Mr. Do will die as soon as any of the unicorns catch him. He can, however, survive a fall of any height, so don't be afraid to jump off a ledge if it will save your life.
These are Mr. Do's nemeses, the dreaded Unicorns (Mini-Dinos). They chase Mr. Do around the castle trying to capture him. If they try to walk over a gap in the floor, they will fall through and become stuck. If left alone like this for some time, they will wiggle themselves back out of the gap and fill the hole in with a fill block. Unicorns can walk over the heads of stuck Unicorns to advance past them. At first, Red Unicorns start out slow. The longer they run around a stage, the faster they become. They increase not only their running speed, but the speed at which they can climb out of gaps. You can swing your hammer to push Unicorns back. When they get wedged inside of a gap in the floor, you can hammer them through the gap, and they become angry. Once they get angry enough, they turn green. Green Unicorns are essentially the same as Red Unicorns, but if you hammer a Green Unicorn through the floor, they evolve into Blue Unicorns (Driller-Dinos). Blue Unicorns are a lot tougher than the Red and Green variety. They can not be pushed back by a hammer swing, or be hammered through a gap. After you spend a long time on a given stage, every Unicorn will instantly evolve into a Blue Unicorn. On top of that, each Blue Unicorn will split and multiply into two Blue Unicorns, doubling their numbers every 30 seconds. It is imperative to destroy the Blue Unicorns or find some other way to get out of the stage once that happens.
There are three key blocks arranged in the floors of every stage. Every time you knock out a key block, the doors at the top of the castle open a little bit, until the last key block is removed and the doors open wide. Behind the doors is the Flag Shield. Reach the top and collect the flag shield to temporarily transform the Unicorns into Alphamonsters. Alphamonsters contain one of the letters in the word "EXTRA" on their chest. Every time you kill an Alphamonster, either by squashing them with you hammer, knocking a block on top of them, or falling and landing on them, you collect one of the letters if you haven't already. The letters you have collected are signified by the flags at the top of the castle. If you manage to collect all five letters, you will be granted an extra life and immediately sent to the next stage. If you do not collect every letter, or clear the stage of every Alphamonster, they will eventually transform back into Unicorns. The walls of the castle will return to their normal color shortly before they transform back, so be aware of this change and stop pursuing them when it occurs.
This ultra rare free-game diamond might appear when you knock a block to the floor. It is entirely random, and a very infrequent occurrence. If you collect it, you will earn 8,000 points, get a credit towards a free game, and immediately advance to the next stage.
There are four types of blocks in Mr. Do's Castle. Understanding each of their functions is crucial to playing Mr. Do's Castle properly.
| Kill a red unicorn with a block that falls one floor | 500 points |
| Kill a red unicorn with a block that falls two floors | 1000 points |
| Kill a red unicorn with a block that falls three floors | 1500 points |
| Kill a red unicorn with a block that falls four floors | 2000 points |
| Kill a green unicorn | points ×2 |
| Kill a blue unicorn | points ×3 |
| Kill 2 unicorns with one block | points ×2 |
| Kill 3 unicorns with one block | points ×4 |
| Kill 4 unicorns with one block | points ×6 |
| Colleting the flag shield | 1000 point + 500 × level |
| Collecting a diamond | 8,000 points. |