Wonder Boy in Monster Land/Round 9

Head right, jumping across the platforms over the lava.

When you enter the cave, jump directly down, and go down five-screens worth of areas. You should eventually see the second small two-tile platform. Open a secret door here to fight a boss which attacks by using a fiery orbs in a circle. Defeat it to collect a new sword.

Head down, and continue to the right. Use the platforms to jump over the water, and enter the first visible door. Defeat the boss here to collect the key to exit the level and into the fortress to the right.