p.
At the first turn, you must select a square for the king anywhere on
first rank. This deviates from the original intention of the author,
- because castling rules would be rather tedious to implement.
- Thus, you can instead "castle on move 1" :-)
+ but allows to "castle on move 1" without requiring a rook in a corner.
p.
If one of your undefined pieces is captured, you first have to choose which