The sequel to Mr. Do!, Mr. Do!'s Castle for the ColecoVision brings Mr. Do! indoors. In the first game your job was to help the little clown harvest his orchard. In this game you must help Mr. Do! restore the sanctity of his castle by assisting him in getting rid of the pesky red, blue and green unicorns that are roaming the hallways. If a unicorn bites you, you lose a life.
There are eight different scenes in Mr. Do!'s Castle. Each scene is composed of seven floors, and each floor is connected by ladders. The middle five floors are composed primarily of blocks. To get rid of the mischievous one-horned creatures, you must use your hammer to knock out a floor block while a unicorn is standing under it. In the first two levels, you can bop the unicorns on the head while they are standing directly on a block. To slow the unicorns down, you can knock out a block and temporarily trap the unicorns in the vacant space.
Since cherries are Mr. Do!'s favorite food, most of the floor blocks have cherries in them. However, three of the blocks contain keys. If you knock out all three key blocks and then run up to the door at the top of the castle, all of the unicorns will briefly turn into bonus letter unicorns. While the unicorns are in this state, you can whack them with your hammer. If you spell out the word EXTRA during the course of the game, you get an extra life.
To finish a stage in this game, you must do one of two things: knock out all of the cherry blocks or eliminate all of the unicorns.
Parker Brothers also produced this game for the Atari 5200. The only substantive difference between the two versions is that in the ColecoVision game, the red unicorns will turn green if you hit them too many times with your hammer, and the greens ones will turn blue if they are hit too many times. In both versions of the game, the blue unicorns can clone themselves.