Mr. Do's Castle/Walkthrough

Clear the stage by killing Unicorns
The most common method of clearing a stage in Mr. Do's Castle is by defeating all of the Unicorns in the stage. There are a couple of ways of doing that, but the most common way is to knock a block on their heads. You can do this either by waiting for one to simply walk underneath your current position, or you can create a hole in one floor, and climb up to a higher floor, and wait for a Unicorn to get stuck in the hole before hammering the block above him. Better yet, you can wait for one Unicorn to get stuck in the floor, and wait for another Unicorn to walk on his head before batting down a block and killing both of them at once.

In the very early levels when the Unicorns are especially slow, it is possible to kill a Unicorn by knocking out the floor under their very feet. By stage 3, this method stops working unless you are extremely lucky with your timing, and cannot be counted on as a method of killing Unicorns. Likewise, it is possible to wait for a Unicorn (including a Blue Unicorn) to climb all the way out of a hole and replace the gap with a fill block, and then smash the fill block at the earliest moment possible. The Unicorns remain still for a split second after filling in a gap, and if you time it right, you can kill them by knocking them to the floor below. This timing is extremely difficult to master, and takes a lot of practice. Too soon, or too early, and the Unicorn will be unaffected and catch you.

Using swing ladders effectively
Unicorns have a general strategy of taking the shortest path that they can find to reach you. Many times, this includes swing ladders that you can push when you stand above them. If you need to escape from chasing Unicorns, you can climb up a swing ladder, and push it away once you get to the top. Just make sure that you don't strand yourself on a ledge with a Unicorn on one end, and Unicorns waiting for you below. Swing ladder are typically position under blocks, so it's possible to wait for Unicorns to climb swing ladders and knock a block down on top of them as they climb.

Using the skull bridge
Proper use of the skull bridge can be one of the most effective methods of killing Unicorns if you can set it up properly. A skull bridge is only formed with both skulls that comprise either end of the bridge are knocked out. Therefore, in order to properly create a skull bridge trap, one of the skull blocks must already be removed. You want to choose the skull block that is on the end that the Unicorns are most likely going to attack you from. If you choose the wrong side, you will be a sitting duck for the Unicorns. After knocking out one skull block, knock out as many blocks between the skull blocks as you have time for. Then wait on the other side of the remaining skull block for the Unicorns to approach. Wait for a few Unicorns to get stuck in the gaps and knock out the last skull block. Every block between the skulls will fall, and the Unicorns that were stuck will die. Any Unicorn that was standing beneath the blocks will die also. It is possible, especially early on in the game, for Unicorns that were simply standing on the bridge when you hit the skull block to die as well, but this never works on Blue Unicorns. Remember that a bridge will be created between the skulls, giving any Unicorns beyond direct access to you, so have an escape plan.

Clear the stage by breaking cherry blocks
Note that this method is only available in later revisions of the game. This can be a particularly difficult method of clearing a stage but it is possible, especially after you've reduced the number of Unicorns that are chasing you. It takes a considerably long time to do this, and by the time you are close to finishing, the remaining Unicorns may become Blue Unicorns and start multiplying. It is best to work from the bottom of the screen to the top because the game is less dangerous at the start of the stage, and you are safer at the top of the stage when the pace picks up. One thing to be mindful of, is to watch out for the Unicorns when they begin to approach you from the top of the screen. Be flexible and be prepared to skip a floor or two until you can come back to it. Since you can fall any height without fear of dying, you can always drop down a floor or two (or more) much faster than the Unicorns can. If you are attempting this method of clearing a stage, you do not have to remove the key blocks or the skull blocks, although knocking out two skull blocks is usually faster than knocking out all of the cherry blocks in between, unless you are attempting the skull bridge trap mentioned above.

Clear the stage by collecting EXTRA
This is another tricky method of clearing a stage, but depending on how many letters you currently have and need, it can be the fastest method of the three. The trick to this method is clearing out all three keys blocks in the stage. You'll have an easier time removing at least one or two if you run directly to the closest ones as soon as the game starts. Getting the last one or two keys may become problematic based on where they keys are and how quickly the Unicorns descend down the ladders. It may be necessary to lure some of the Unicorns away from the last key or keys before you attempt to hit them. Once all three keys are smashed, the door at the top will open wide, the Flag Shield at the top will flash, and you will hear a chime. Now you have to safely maneuver Mr. Do to the top of the castle. If you make it there, do not immediately rush over and grab the shield. Instead, wait as close as you possibly can without touching it, and wait for at least one Unicorn (if not more) to get as close to you as you dare allow before grabbing the shield. This way, you will have access to at least one easy Alphamonster that you can smash before pursuing the rest.

The Unicorns won't remain Alphamonsters for long, and they have an unfortunate habit of clinging to the side ladders where you can't drop blocks on them. Chase after them by creating and falling through holes in the floor, you fall faster than you can climb. It is still possible to hammer them when they're on the ladder. In order to hit them with your hammer, you have to be very close to them. If there is a gap between you and the Alphamonster, you are more likely to miss, giving them a chance to get farther away from you. They also don't remain one letter all the way through. They advance through the letters "EXTRA" in that order, e.g. after a while, the X letter will become the T letter. So if you need the letter X, and an X is far away from you, look for an E that might be closer and wait for it to become an X before you hit it.