diff --git a/js/pacosako.js b/js/pacosako.js
index 474929c..91683c3 100644
--- a/js/pacosako.js
+++ b/js/pacosako.js
@@ -40,8 +40,9 @@ const COLUMNS = 'abcdefgh';
  * effect when the game was started.
  *
  * Version 1: Initial version. (Default if version field is missing.)
+ * Version 2: Prohibit moving through check.
  */
-const CURRENT_VERSION = 1;
+const CURRENT_VERSION = 2;
 
 function otherSide(side) {
    if (side === LIGHT) {
@@ -622,7 +623,18 @@ class Game {
                   board.isEmpty('f' + row) &&
                   board.isEmpty('g' + row)) {
                if (!this.isInCheck(side)) {
-                  yield 'g' + row;
+                  if (this._version < 2) {
+                     yield 'g' + row;
+                  } else {
+                     /* Prohibit castling through check */
+                     const testGame = new Game(this);
+                     testGame._board.putPiece(side, 'e' + row, EMPTY);
+                     testGame._board.putPiece(side, 'f' + row, KING);
+                     testGame._checkCache = {};
+                     if (!testGame.isInCheck(side)) {
+                        yield 'g' + row;
+                     }
+                  }
                }
             }
 
@@ -631,7 +643,18 @@ class Game {
                   board.isEmpty('c' + row) &&
                   board.isEmpty('b' + row)) {
                if (!this.isInCheck(side)) {
-                  yield 'c' + row;
+                  if (this._version < 2) {
+                     yield 'c' + row;
+                  } else {
+                     /* Prohibit castling through check */
+                     const testGame = new Game(this);
+                     testGame._board.putPiece(side, 'e' + row, EMPTY);
+                     testGame._board.putPiece(side, 'd' + row, KING);
+                     testGame._checkCache = {};
+                     if (!testGame.isInCheck(side)) {
+                        yield 'c' + row;
+                     }
+                  }
                }
             }
          }