Fixes...

src/de/steamwar/towerrun/game/TowerRunGame.java

@@ -58,7 +58,7 @@ public class TowerRunGame {
 
             for (Location door : WorldConfig.DOORS) {
                 door.getBlock().setType(Material.AIR);
-                door.add(0, 1, 0).getBlock().setType(Material.AIR);
+                door.clone().add(0, 1, 0).getBlock().setType(Material.AIR);
             }
         } else {
             throw new IllegalStateException("Game is already running!");

src/de/steamwar/towerrun/listener/IngameListener.java

@@ -34,6 +34,7 @@ import org.bukkit.event.block.Action;
 import org.bukkit.event.block.BlockBreakEvent;
 import org.bukkit.event.block.BlockPlaceEvent;
 import org.bukkit.event.entity.EntityDamageByEntityEvent;
+import org.bukkit.event.entity.EntityRegainHealthEvent;
 import org.bukkit.event.entity.ItemSpawnEvent;
 import org.bukkit.event.entity.PlayerDeathEvent;
 import org.bukkit.event.player.PlayerInteractEvent;
@@ -114,4 +115,11 @@ public class IngameListener extends GameStateBukkitListener {
     public void onPlayerRespawn(PlayerRespawnEvent event) {
         event.setRespawnLocation(WorldConfig.SPAWN);
     }
+
+    @EventHandler
+    public void onEntityRegainHealth(EntityRegainHealthEvent event) {
+        if (event.getEntityType() == EntityType.PLAYER) {
+            event.setCancelled(true);
+        }
+    }
 }