A Phantom Block is a place marker for a brick will be put later on. The phantom blocks are used for bricktrons to know where to build next. A builder bricktron will continue to build and replace the phantom blocks with bricks until the stockpile is depleted. When the stockpile is depleted you will need to gather additional resources to make more bricks and continue building. You can also make a blueprint with phantom blocks called prefabs, which is used to save a model. A prefab's size is limited, so you cannot save your entire castle as a blueprint. The first appearance of the phantom block was in the "Castle Story - First Demonstration" video that was published in December 2011. They appear in blue.