The original La-Mulana was known as a "Metroidvania"-style game (a 2D action platformer with an emphasis on non-linear exploration). That said, the real core of the game is in deciphering the riddles scattered throughout the game, and solving the puzzles that exist as part of the ruins.
By deciphering cryptic text on the monuments scattered throughout the ruins, you will gradually unravel the mystery of the ruins. Which parts of the sprawling underground you tackle, and in what order, is up to you. Sometimes you will need to use changes in backgrounds, mysterious sounds, the strength of enemies, or the sneakiness of traps to detect subtle changes in your surroundings.
Plenty of ferocious and terrifying monsters and guardians stand in your way in the depths of this new underground network. Fortunately, you will also have an arsenal of tools and weapons at your disposal to help you on your journey.