[[caption-width-right:264:Leo and Purapril from ''Monster Lair''.]]
''Wonder Boy III: Monster Lair'', sometimes shortened to simply ''Monster Lair'', is the third game in the "Franchise/WonderBoy" series. An auto-scrolling {{platform game}}/{{shoot em up}} hybrid, it is the only game in the series to offer 2-players co-op play. Players control either the Leo or Purapril, as they set off to follow in the footsteps of Bocke Lee Temjin and free their land from a monster invasion.

Originally released in 1988, it was the last "Wonder Boy" game released for the arcades and saw home versions for the UsefulNotes/TurboGrafxCD in 1989 and the UsefulNotes/MegaDrive in 1990.

!!Tropes in ''Monster Lair'':
* AndYourRewardIsClothes: Once you beat the game, you get to play through again wearing the Legendary Armor, with a higher difficulty level.
* BeeAfraid: Round 4 and its boss.
* BigBoosHaunt: Round 9 and 13.
* BigFancyCastle[=/=]TempleOfDoom: The sixth stage. The prior stage has you [[DeathMountain ascending a mountain]] to reach the castle.
* BlindIdiotTranslation: The arcade version's ending text. "The invaders from the space were destroyed by your courageous fight..." The Mega Drive's ending screen isn't much better.
* CyberCyclops: The Round 13 boss.
* DegradedBoss: The cactus-o-lantern boss returns as a GiantMook in Round 13.
* DirectContinuousLevels: Rounds 4-6, 9-10, and 11-14.
* DubInducedPlotHole: The manual for the UsefulNotes/TurboGrafx16 version calls the protagonist Adam, but the high scores still show his name as Leo. Worse, the manual lists the Legendary Armor as the "Legendary Weapon of Complete Destruction".
* EternalEngine / SpaceZone: Level 14.
* EvilDoppelganger: At first, the final boss appears to be Book the Hero or the original Wonder Boy (hence his name in the title), but it turns out to be aliens.
* GameplayRoulette: The first half of each round is AutoScrolling platforming, the second half, where you fight the boss, is ShootEmUp.
* GaidenGame / AlternateContinuity: This picks up from the ending of the arcade version of ''Monster Land'', where aliens steal the Legendary equipment. This is inconsistent with the SMS sequel, where Book the Hero [[BagOfSpilling leaves it behind]] in the collapsing castle after being cursed, but regains it near the end of the game.
* GreenHillZone: Round 4
* HardModeFiller: Stages 11, 12, and 13 are mainly rehashes of Stages 1, 5 (with a touch of 7's SlippySlideyIceWorld) and 9 (also iced over) respectively.
* IcePalace: Round 13.
* JungleJapes: Round 2.
* KingMook: About half the bosses, including a giant snake, a giant bat, the Queen Wasp, a cactus jack-o-lantern, a GiantEnemyCrab, etc. The King Demon from ''Monster Land'' also returns.
* LongSongShortScene: Most of the stages, as well as the boss battles, end before you get to hear the entirety of the music, at least in the [=TurboGrafx-CD=] version.
* MarketBasedTitle: The [=TurboGrafx-CD=] version drops the ''Wonder Boy III'' part of the title and is simply called ''Monster Lair''. This does not apply to the PC Engine version, which still used the arcade game's full title.
* MonsterTown: Round 10 is a village inhabited by the Myconid mushroom mooks from ''Monster Land'', and the boss is a giant version.
* OneWingedAngel: The BigBad and FinalBoss reveals his grotesque true form after you slay his dragon.
* PalmtreePanic: Rounds 1 and 11.
* RocketPunch: Round 13's boss has this as ThatOneAttack.
* SegmentedSerpent: The boss of Round 2.
* ShiftingSandLand: Level 8.
* SlippySlideyIceWorld: Levels 7 and 12.
* SpellMyNameWithAnS: Purapril is also called "Pappilo" and "Papillo" in the European arcade flyer, and "Pricilla" in the Mega Drive version (Purapril is an accurate official translation of her name that comes her apparent descendants in ''[[VideoGame/WonderBoyInMonsterWorld Wonder Boy in Monster World]]'' and ''VideoGame/MonsterWorldIV''). In the UsefulNotes/TurboGrafxCD version, the playable characters were instead called "Adam" and "Laura".
* UndergroundLevel: Level 3.
* WizardNeedsFoodBadly: Just like the first game.