The two tall stacks of blocks on the left are what you are suppose to
build. A set of identical blocks are laying on the ground and the user
has started building them on the right side. The user clicks and drags the
blocks to put them together.
In 2011, the world was modified by adding an event for each block. There is an invisible block at the correct location for each block. The event just checks to see if the block's distance to the correct invisible block is less than 1 and if so, it moves the block to the invisible block, snapping it into the correct location.