Arrange the numbers from 1 to 8 in order from left to right. Exact formation patterns differ by variant.
(v1.5 update: Now includes numbers from 1 to 12, in a 6x6 board.)

Two Pieces may be switched, or castled, in any direction (up/down, left/right, and diagonally). Castle-switch tiles according to their corresponding number values. For example:
a '5' will castle with an immediately adjacent 4 or 6.
a '5' will with a 3 or 7 - at two spaces away.
a '5' will with a 2 or 8 - at three spaces away.
Other numbered Pieces are likewise moved.