>>720090785
That would be the Deku Palace.
After saving the witch, a bunch of monkeys approached you and asked you to help save their friend who is trapped there.
After visiting the palace throne and seeing the captive, you'll be approached by monkeys again and they'll point you toward what you have to do to progress in the swamp. You'll be directed to sneak through the eastern palace garden (much easier to do at night) and purchase some magic beans. After a Deku Link platforming segment, you'll receive a key item and further direction.
I don't blame you for getting lost. MM is a bit more complicated than the average Zelda. I got stuck many, many times the first time I played it. I even did the same thing as you and tried focusing on side quests because I thought one might be hiding some sort of vital item that I was missing. Off the top of my head, I can only think of one time where this is actually the case, and you weren't/aren't close to that part yet.
