After you get the Eye of the Graeae, you can get a token of love, which allows you to get a key.
Go to Argolis
After you get the Eye of the Graeae, go to the left, and jump up onto the ledge to find a door. Go through it and follow the path to return to the first door of Laconia. Go to the right from there until you reach the Sun Monument. Press START and equip the harp, then press SELECT to use it. The Pegasus will take you back to Arcadia.
Go to the white house to get a password from Zeus, then go back until you are outside of the white house. Go to the left until you reach the last tree door before the end of the forest. Go inside and walk past the old woman to reach the part of the forest that leads to Argolis. Go to the left and enter the small house to reach Argolis.
Get the Token of Love
In Argolis, go to the right until you reach a dead end. Go into the door on the left, then go to the right until you are to the left of a door. Press START to equip the Eye of the Graeae, then press SELECT to use it. Go into the door that appears, and go to the right to reach a new area.
In this new area, go to the left and jump onto the ledge, then keep going left along the ledges until you reach a door. Go in, and go to the right. You will find a Nymph. She will give you a token of Helene's love, and a letter from Helene.
Get the Key
Now that you have the token of love, go to the left, then go to the left again. When you are back in the large cave, go to the right, and go into the first door that you reach after going down from the ledge. Keep going to the left until you are back in the first cave. Go to the left and go through the door there.
Go to the left until you're outside, and use the harp at the Sun Monument to summon the Pegasus, which takes you to the nectar fountain at Athens. Fill up your health, then go to the right and enter Athena's temple to get a password from her. Then leave her temple, and go to the left and enter the large house beyond the nectar fountain. Talk to Keleos, and since you have the token of love, he will give you a Key.
With the key, you will now be able to reach the Nymph who has the next token of love.