diff client/climisc.c client/climisc.c
--- client/climisc.c	Fri Jul 28 04:16:31 2000
+++ client/climisc.c	Sat Aug 26 00:51:00 2000
@@ -366,6 +366,8 @@
 void client_diplomacy_clause_string(char *buf, int bufsiz,
 				    struct Clause *pclause)
 {
+  struct city *pcity;
+
   switch(pclause->type) {
   case CLAUSE_ADVANCE:
     my_snprintf(buf, bufsiz, _("The %s give %s"),
@@ -373,9 +375,20 @@
 		advances[pclause->value].name);
     break;
   case CLAUSE_CITY:
-    my_snprintf(buf, bufsiz, _("The %s give %s"),
-		get_nation_name_plural(pclause->from->nation),
-		find_city_by_id(pclause->value)->name);
+    pcity = find_city_by_id(pclause->value);
+    if (pcity) {
+      my_snprintf(buf, bufsiz, _("The %s give %s"),
+                  get_nation_name_plural(pclause->from->nation),
+		  pcity->name);
+    } else {
+      /* FIXME: We end in here when
+         1. Player A have seen city owned by player B.
+         2. Player C conquers city, but player A don't know this because of
+            fog of war -> Player A have old city id.
+         3. Players A and B meet.
+         4. Player A adds clause asking city. */
+      my_snprintf(buf, bufsiz,_("Known bug: Impossible clause, just erase."));
+    }
     break;
   case CLAUSE_GOLD:
     my_snprintf(buf, bufsiz, _("The %s give %d gold"),
diff common/diptreaty.c common/diptreaty.c
--- common/diptreaty.c	Tue May 30 01:10:22 2000
+++ common/diptreaty.c	Fri Aug 25 23:35:02 2000
@@ -14,6 +14,7 @@
 #include <stdlib.h>
 
 #include "genlist.h"
+#include "log.h"
 #include "mem.h"
 #include "player.h"
 
@@ -80,15 +81,6 @@
       /* same clause already there */
       return 0;
     }
-    if(type==CLAUSE_GOLD
-       && pclause->type==CLAUSE_GOLD
-       && pclause->from==pfrom) {
-      /* gold clause there, different value */
-      ptreaty->accept0=0;
-      ptreaty->accept1=0;
-      pclause->value=val;
-      return 1;
-    }
     if(is_pact_clause(type) &&
        is_pact_clause(pclause->type)) {
       /* pact clause already there */
@@ -96,6 +88,25 @@
       ptreaty->accept1=0;
       pclause->type=type;
       return 1;
+    }
+    switch(type) {
+    case CLAUSE_ADVANCE:
+      if (pclause->value < A_FIRST || pclause->value >= A_LAST) {
+        freelog(LOG_ERROR, "Out of range tech value %i in clause.",
+	        pclause->value);
+        return 0;
+      }
+      break;
+    case CLAUSE_GOLD:
+      if (pclause->type==CLAUSE_GOLD && pclause->from==pfrom) {
+        /* gold clause there, different value */
+        ptreaty->accept0=0;
+        ptreaty->accept1=0;
+        pclause->value=val;
+        return 1;
+      }
+      break;
+    default:
     }
   }
    
diff server/diplhand.c server/diplhand.c
--- server/diplhand.c	Fri Aug 25 09:53:49 2000
+++ server/diplhand.c	Sat Aug 26 00:55:50 2000
@@ -73,7 +73,8 @@
 				    struct packet_diplomacy_info *packet)
 {
   struct Treaty *ptreaty;
-  struct player *plr0, *plr1, *pgiver, *pdest;
+  struct player *plr0, *plr1, *pgiver, *pdest, *other;
+  int *giver_accept;
   
   plr0=&game.players[packet->plrno0];
   plr1=&game.players[packet->plrno1];
@@ -81,9 +82,66 @@
   
   if((ptreaty=find_treaty(plr0, plr1)) && pgiver==pplayer) {
     if(ptreaty->plr0==pgiver)
-      ptreaty->accept0=!ptreaty->accept0;
+      giver_accept = &ptreaty->accept0;
     else
-      ptreaty->accept1=!ptreaty->accept1;
+      giver_accept = &ptreaty->accept1;
+
+    other=(plr0==pplayer) ? plr1 : plr0;
+    if (! *giver_accept) { /* Tries to accept. */
+
+      /* Check that player who accepts can keep what (s)he promises. */
+
+      struct genlist_iterator myiter;
+
+      genlist_iterator_init(&myiter, &ptreaty->clauses, 0);
+      
+      for(;ITERATOR_PTR(myiter); ITERATOR_NEXT(myiter)) {
+	struct Clause *pclause=(struct Clause *)ITERATOR_PTR(myiter);
+        struct city *pcity;
+
+	if(pclause->from == pplayer) {
+          switch(pclause->type) {
+          case CLAUSE_ADVANCE:
+            if (get_invention(pplayer, pclause->value) != TECH_KNOWN) {
+              freelog(LOG_ERROR,
+                    "The %s don't know tech %s, but try to give it to the %s.",
+                      get_nation_name_plural(pplayer->nation),
+		      advances[pclause->value].name,
+                      get_nation_name_plural(other->nation));
+              notify_player(pplayer,
+                   _("Game: You don't have tech %s, you can't accept treaty."),
+                            advances[pclause->value].name);
+              return;
+            }
+            break;
+          case CLAUSE_CITY:
+            pcity = find_city_by_id(pclause->value);
+            if (!pcity || pcity->owner != pplayer->player_no) {
+	      notify_player(pplayer,
+                        _("You are not owner of %s, you can't accept treaty."),
+                            pcity->name);
+              return;
+	    }
+            if (pcity && pcity->improvements[B_PALACE]) {
+              notify_player(pplayer,
+             _("Game: Your capital is requested, you can't accept treaty."));
+              return;
+	    }
+            break;
+          case CLAUSE_GOLD:
+            if (pplayer->economic.gold < pclause->value) {
+              notify_player(pplayer,
+             _("Game: You don't have enought gold, you can't accept treaty."));
+              return;
+            }
+            break;
+          default:
+	  }
+	}
+      }
+    }
+
+    *giver_accept = ! *giver_accept;
       
     lsend_packet_diplomacy_info(&plr0->connections, 
 			       PACKET_DIPLOMACY_ACCEPT_TREATY, 
@@ -112,23 +170,45 @@
 	      get_nation_name_plural(plr0->nation),
 	      get_nation_name_plural(plr1->nation));
       
+      /* Check that one who accepted treaty earlier still have everything
+         (s)he promised to give. */
       
-      /* verify gold! the player's gold amount could have changed during
-       * the meeting */
       genlist_iterator_init(&myiter, &ptreaty->clauses, 0);
       for(;ITERATOR_PTR(myiter); ITERATOR_NEXT(myiter)) {
 	struct Clause *pclause=(struct Clause *)ITERATOR_PTR(myiter);
-	pgiver=pclause->from;
-
-	if(pclause->type==CLAUSE_GOLD && pgiver->economic.gold<pclause->value) {
-	  
-	  notify_player(plr0, _("Game: The %s don't have the promised amount "
-				"of gold! Treaty canceled!"),
-			get_nation_name_plural(pgiver->nation));
-	  notify_player(plr1, _("Game: The %s don't have the promised amount "
-				"of gold! Treaty canceled!"),
-			get_nation_name_plural(pgiver->nation));
-	  goto cleanup;
+        struct city *pcity;
+        if (pclause->from == other) {
+	  switch(pclause->type) {
+          case CLAUSE_CITY:
+            pcity = find_city_by_id(pclause->value);
+            if (!pcity || pcity->owner != other->player_no) {
+	      notify_player(plr0,
+                            _("Game: The %s no longer control %s! "
+                              "Treaty canceled!"),
+                            get_nation_name_plural(other->nation),
+                            pcity->name);
+	      notify_player(plr1,
+                            _("Game: The %s no longer control %s! "
+                              "Treaty canceled!"),
+                            get_nation_name_plural(other->nation),
+                            pcity->name);
+              goto cleanup;
+	    }
+	  case CLAUSE_GOLD:
+            if (other->economic.gold<pclause->value) {
+	      notify_player(plr0,
+                            _("Game: The %s don't have the promised amount "
+                              "of gold! Treaty canceled!"),
+                            get_nation_name_plural(other->nation));
+	      notify_player(plr1,
+                            _("Game: The %s don't have the promised amount "
+                              "of gold! Treaty canceled!"),
+                            get_nation_name_plural(other->nation));
+              goto cleanup;
+	    }
+            break;
+          default:
+	  }
 	}
       }