diff ai/aihand.c ai/aihand.c
--- ai/aihand.c	Tue Dec 26 22:50:48 2000
+++ ai/aihand.c	Mon Jan  1 15:51:12 2001
@@ -335,7 +335,7 @@
             } else defender = punit;
           } else if (incity->shield_surplus > 0) {
             pack.unit_id = punit->id;
-            pack.city_id = incity->id;
+            pack.city_id = incity->orig_id;
             handle_unit_change_homecity(pplayer, &pack);
             city_refresh(pcity);
 	    freelog(LOG_VERBOSE, "Reassigning %s from %s to %s",
diff ai/aiunit.c ai/aiunit.c
--- ai/aiunit.c	Sat Dec 30 16:18:05 2000
+++ ai/aiunit.c	Mon Jan  1 15:51:12 2001
@@ -460,7 +460,7 @@
     /* FIXME: it is stupid to change homecity if the unit has no homecity
        in advance or the new city does not have enough shields to support it */
     packet.unit_id = punit->id;
-    packet.city_id = pcity->id;
+    packet.city_id = pcity->orig_id;
     handle_unit_change_homecity(unit_owner(punit), &packet);
     return 1;
   }
@@ -1617,7 +1617,7 @@
        * -AJS, 990704
        */
 	req.unit_id = punit->id;
-	req.city_id = pcity->id;
+	req.city_id = pcity->orig_id;
 	handle_unit_help_build_wonder(pplayer, &req);
       }
     }
@@ -1643,7 +1643,7 @@
            auto_settler_do_goto(pplayer,punit, pcity->x, pcity->y);
          } else {
            req.unit_id = punit->id;
-           req.city_id = pcity->id;
+           req.city_id = pcity->orig_id;
            handle_unit_establish_trade(pplayer, &req);
         }
       }
@@ -2097,7 +2097,7 @@
     if (pdiplomat->homecity != pcity->id) {
       /* this may be superfluous, but I like it.  -AJS */
       req.unit_id=pdiplomat->id;
-      req.city_id=pcity->id;
+      req.city_id=pcity->orig_id;
       req.name[0]='\0';
       handle_unit_change_homecity(pplayer, &req);
     }
@@ -2181,7 +2181,7 @@
       did = -1;
       if ((dist == 1) && (pplayer->player_no != ctarget->owner)) {
 	dact.diplomat_id=pdiplomat->id;
-	dact.target_id=ctarget->id;
+	dact.target_id=ctarget->orig_id;
 	if (!pdiplomat->foul && diplomat_can_do_action(pdiplomat,
 	     DIPLOMAT_EMBASSY, ctarget->x, ctarget->y)) {
 	    did=pdiplomat->id;
diff server/citytools.c server/citytools.c
--- server/citytools.c	Mon Jan  1 15:37:50 2001
+++ server/citytools.c	Mon Jan  1 15:53:18 2001
@@ -1203,6 +1203,16 @@
      This is fast way to check city id. */
   pcity = find_city_by_id(id);
   if(pcity) {
+
+    /* We will not accept anything but original id from client.
+       This removes some possibilities to cheat. --Caz */
+    if (id != pcity->orig_id) {
+      freelog(LOG_ERROR, "player_find_city_by_orig_id() got current id %i "
+              "for %s, when original id is %i.", id, pcity->name,
+              pcity->orig_id);
+
+      return 0;
+    }
     if (owner && owner->player_no != pcity->owner)
       return 0;
     return pcity;