Capturing is compulsory.

When more than one capture is available, the player may exercise choice.

The king has no special prerogative and accordingly:

It may be captured like any other piece.

There is no check or checkmate.

There is no Castling.

Pawns may also promote to King.

(in the game above there is check and you cannot take the king due to the softwere not being able to understand the rules of antichess)