Many NPCs populate the realm of Gielinor. Although some NPCs, such as shopkeepers, have game functions that require them to be unavailable for combat, most NPCs can be attacked. Attackable NPCs are generally referred to as monsters, regardless of their race. Monsters range from common, low level creatures like chickens and bears to unique, much more powerful monsters like the King Black Dragon, Chaos Elemental or Kalphite Queen. Each type of monster has its own strengths and weaknesses. Demons, for example, have low defence against magical attacks while most dragons have extremely high defence against magic. Monsters may either be aggressive or non-aggressive. The aggressive monsters attack all players who are under twice the combat of the monster. However, in the wilderness, aggressive monsters attack all players regardless of level. Non-aggressive monsters do not attack players until the player attacks first. This can make certain areas throughout Gielinor dangerous or inconvenient to players depending upon their combat levels. RuneScape also features independent mini-games for its players. Mini-games take place in certain areas and normally involve a specific skill. Mini-games usually require players to cooperate or to compete with each other. Popular mini-games include Castle Wars, which is similar to the real-life game Capture the Flag, and Pest Control, a highly combat-focused mini-game. Currently, all mini-games, with the exception of the Stronghold of Security, are available only to paying members. Players can fight against other players in player versus player (PvP) combat in certain areas of Gielinor. The most common place is in the Wilderness, where players risk their lives and their items fighting other players. Players can engage in PvP combat without risking their items in mini-games such as the Duel Arena, Castle Wars, and the TzHaar Fight Pits, and in combat arenas or dungeons in player owned houses. Players engaging in PvP combat in the Duel Arena can agree on rules for the duel such as allowing only melee attacks or preventing players from using potions or food during the fight.