It adds yet another dimension to the rich strategy.
The pieces
The playing pieces are designed to be easily stacked, and easily picked up from the top of a stack. Pieces in a stack can all be of the same color, or they can be of different colors. (For ease of discussion, a single piece on a space is still considered a "stack".)
Control
The person whose piece is on top of a stack is said to have control of that stack. Control determines who is allowed to move pieces from that stack.
Here are various stacks where the light piece is in control:
Here are various stacks where the dark piece is in control:
Unstacking
Once the pieces are stacked up, then what do you do? Pieces may be removed from the top of the stack by the player who has control of that stack. Pieces are always removed as a group from the top of the stack, without rearranging.
Limits
In any game there must be limits. Imaging what would happen if all sixty-four pieces were stacked onto one space! Now imagine trying to move that stack!! It would be physically unworkable, and besides, unlimited stacks would make the game less interesting.
So, to keep things reasonable, there is a limit of eight (8) pieces (in any color combination) on any given space at any time. In addition, there is a limit of four (4) pieces that may be unstacked at a time. These limits, while physically necessary, also become opportunities for further strategy, as will be seen later.