Gameplay
Each person's turn begins with the player "going into a room" by Opening a Door (often referred to as kicking down the door) by drawing a Door card face-up. If there is a monster in the room, the player fights the monster. If the player's level plus bonuses from the player's equipment (such as Really Impressive Title) is higher than the monster's level plus any bonuses the monster might have (such as Enraged, Humongous, or Buffed), then the player wins the fight and moves up one level (though some monsters grant two levels), and takes the monster's stuff. If the drawn card is a curse card, it takes effect immediately. If the player did not find a monster in the room, then the player can choose to either draw another Door card face down (looting the room) or fight a monster from his hand (looking for trouble). To prevent opponents from achieving the winning level (9, 10, 11, 20, or 22 depending on pre-game selections and card play), players can give enhancing cards (such as the Big Honkin' Sword of Character Whupping) to whatever monsters are fighting the other player so that the monsters will win and cause the player to have to try to Run Away from the monsters and maybe have to suffer "Bad Stuff" from the monsters, or throw curses on each other (or have them happen randomly), such as New Edition Rules (causing all players to lose a level). Players can also use items against each other such as Itching Powder (making the player throw away any clothing or armor). Every card played resolves instantly, with few exceptions (which include the use of the card Wishing Ring to cancel curses).
Players can help each other defeat monsters, adding together their level and bonuses to beat the monsters. The player who helps the other player can negotiate a deal to receive some of the Treasure cards earned by defeating the monster, or some other advantageous trade, but the helper never gets a Level for helping without playing a card or using an ability that allows it (e.g., the Elf Race, mentioned below). Players can gain extra abilities or advantages by getting "Class" or "Race" cards; as an example, players using the Warrior class win a battle in the event of a tie between their and the monster's level and pluses, while a player using the Elf race gains a Level per monster whenever they assist another player in killing a monster. Certain monsters, such as Squidzilla, gain an advantage against certain races or classes.
Players can Sell one or more of their items to gain a level. Each item card has a value saying how much gold the item is worth. If the combined value is greater than or equal to 1000 gold pieces, then the items can be sold to gain a level. Since the game has no other way to represent money, players cannot get "change." However, a player can buy more than one level, at a cost of 1000 gold pieces per level. Players cannot achieve the winning level by selling items, however, nor can they sell Items and not take a level if the next level is one that has to be earned by killing a monster (usually the winning level).
Winning the game requires getting to Level 10 (or 20). Players can get levels by killing Monsters, selling Items (as described above) or playing cards that let a player go up a level (such as Bribe the DM or Switch Character Sheets). With few exceptions, the only way to get the winning level is to kill a Monster. Exceptions to that rule usually come in the form of cards which specifically state they break the rule (e.g., Divine Intervention).
Munchkin is not a very serious game; the rules make this clear with phrases like: "Decide who goes first by rolling the dice and arguing about the results and the meaning of this sentence and whether the fact that a word seems to be missing any effect." and "Any disputes in the rules should be settled by loud arguments with the owner of the game having the last word." There are many cards which interact with or are affected by a single other card, despite the rarity of the two cards entering play together (such as the interaction between Fowl Fiend and Chicken on Your Head or Sword of Slaying Everything Except Squid and Squidzilla).
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