Monster Lair is a unique game in the Wonder Boy series. You play Leo, a green-haired character who must stop invaders from collecting weapons and destroying the land. The game starts off like a simple platformer but transforms into a shoot 'em up when you enter the skull further on. This means the gameplay is split between action and shooter scenes.
In the action scenes, your health decreases as you approach the skull. However, you can restore it by collecting fruit. You can pick up various weapons that not only allow you to use them for a limited time but also increase your health.
In the shooter scenes, you ride on the back of a pink creature through the levels. In this section, your health remains steady unless you're hit by an enemy passing by. There's always a boss waiting at the end that you must defeat. The color of the boss changes to show how much damage it has taken, and some bosses have two stages.
If your health drops low in each scene, you'll lose a life. You can also choose to play with Wonder Girl, as the game allows for two-player mode.