To Castle in Chesspark you need to complete the following moves:
White - Castling kingside- Move the King from e1 to g1. Castling queenside - Move King from e1 to c1
Black - Castling kingside- Move the King from e8 to g8. Castling queenside - Move the King from e8 to c8
Castling is only permissible if all of the following conditions hold:
1. The player must never have moved the king;
2. The player must never have moved the rook involved in castling;
3. There must be no pieces between the king and the rook;
4. The king may not currently be in check.
5. The king cannot pass through squares that are under attack by enemy pieces.
6. As with any move, castling is illegal if it would place the king in check.
7. The king and the rook must be on the same rank.
It is a common mistake to think that the requirements for castling are even more stringent than the above. To clarify:
1. The king may have been in check previously, as long as it isn't in check at the time of castling.
2. The rook involved in castling may be under attack.
3. The rook involved in castling may move over an attacked square (This situation is only possible with queenside castling).
Conditions taken from Wikipedia.