Copyright (c) 2004 Dieter Stein

This game is played on the following board.

STACK - A stack of 1+ stones of either color.
The color of the stack's top stone determines its owner (e.g., a black top stone makes it a black stack).
A stack is safe if it has three enemy stones.
CASTLE - Each player has a castle, which is the area where his army begins.
TURN - On each turn, each player must move one of his stacks. A stack may:
Slide - A stack with N friendly stones can slide up to N empty cells in a straight line.
A stack cannot jump, but can finish its slide on top of another (friend or foe) non-safe stack, provided that there are not more than three pieces of the same color in the resulting stack.
Split - Any set of upper stones may be split and slide independently of the rest.
This split may be executed several times, while there's some part of the original stack left.
If and when the enemy releases an enemy stack, his turn is over. 
It is forbidden to release an enemy stack inside the player's own castle.
The moving range of a piece is determined looking at its stack position before actually moving (excluding any enemy piece). The bottom piece may move 1 cell, the second highest piece may move 1 or 2, and the upper piece may move 1, 2 or 3 cells.
GOAL - Wins the player that places three stacks inside the opponent's castle.
An example [taken from the game website]

[Castle's cells are identified with blue dots. Check coordinates by moving the mouse over the board cells] 

Black must defend the White attack from f6 to his stack at d6 (White would create a safe stack inside Black's castle). He can easily defend by moving, one cell sideways, the stack at d5 to e6.

Another Black option is to stack d6 into d5 and make a double threat: (a) to slide d5 to g5 creating a safe stack, (b) it frees the triple black stack at c6 to capture f6.

This game is a variant of Accasta. Some notes from the author: Accasta is an abstract board game for two players. Influenced by Dr. Emanuel Lasker's Lasca and Wladyslaw Glinksi's Hexagonal Chess, I tried to achieve a clean and original game using stacks and a hexagonal board.  In spring 1998, after a long time of development, Accasta was ready to be published on the Web and in the German game inventor's magazine Spiel & Autor, Vol. 34. Accasta took part in some game design competitions and was one of the finalists of the Premio Archimede 1998 in Italy. In 2001 the first version for online play was implemented for Richard's PBeM Server. [...] In the Pari variant it is obviously a good strategy to stack one's own pieces - even more as in the standard game, as it leads not only to more powerful stacks (regarding multiple moves), but also promotes the moving piece. On the other hand, it is still true that powerful stacks are in danger to turn into safe stacks controlled by the opponent.