diff -r d20887f0e8b3 -r d0c3d3016650 src/chess/fen.c
--- a/src/chess/fen.c	Thu Sep 03 15:28:54 2026 +0200
+++ b/src/chess/fen.c	Thu Sep 03 16:18:24 2026 +0200
@@ -77,40 +77,12 @@
     return 1;
 }
 
-static bool fen_castling_chkmoved(GameState *gamestate,
-    Rank rank, File file) {
-
-    for (unsigned i = 0 ; i < gamestate->movecount ; i++) {
-        if (gamestate->moves[i].fromfile == file
-            && gamestate->moves[i].fromrank == rank) {
-            return true;
-        }
-    }
-
-    return false;
-}
-
 static size_t fen_castling(char *str, GameState *gamestate) {
-    bool K, Q, k, q;
-
-    if (fen_castling_chkmoved(gamestate, rankidx('1'), fileidx('e'))) {
-        K = Q = false;
-    } else {
-        K = !fen_castling_chkmoved(gamestate, rankidx('1'), fileidx('h'));
-        Q = !fen_castling_chkmoved(gamestate, rankidx('1'), fileidx('a'));
-    }
-    if (fen_castling_chkmoved(gamestate, rankidx('8'), fileidx('e'))) {
-        k = q = false;
-    } else {
-        k = !fen_castling_chkmoved(gamestate, rankidx('8'), fileidx('h'));
-        q = !fen_castling_chkmoved(gamestate, rankidx('8'), fileidx('a'));
-    }
-
     size_t i = 0;
-    if (K) str[i++] = 'K';
-    if (Q) str[i++] = 'Q';
-    if (k) str[i++] = 'k';
-    if (q) str[i++] = 'q';
+    if (!gamestate->castling.K) str[i++] = 'K';
+    if (!gamestate->castling.Q) str[i++] = 'Q';
+    if (!gamestate->castling.k) str[i++] = 'k';
+    if (!gamestate->castling.q) str[i++] = 'q';
     if (!i) str[i++] = '-';
 
     return i;