Minecraft Dungeons

This can be found in the hub world just to the left of the tower start door. You will need the artifact "Light Feather" as it will allow you to hop to where is needed.

You will stand on the 2 little stairs to the left of the door around the corner (basically on the grass to the left of the tower door). Face the wooden railing to the left and using the feather "hop" onto the railing. From there fall onto the roof with the chimney. To continue further down go to the bottom left corner of that roof and roll towards the wall diagonally as to fall onto the roof below. (Rolling left too far will cause you to fall off the map). From there continue to the right to the next lowest roof. You should see a platform of wood below this roof go to the back right and walk off carefully to the platform below. Then walk thru the door and in that room will be your achievement.

-Added my own gameplay below-

*As discovered by xx Kenshia xx*
The item isn't needed if you roll off the left of bridge just before before the tower by the two steps at the end of then bridge. There by falling onto the thin wooden ledge and then falling onto the next thin wooden ledge/railing below that. From there you are on the lowest roof and can fall below to the secret door. If you fell inside the second railing area you'll have to use the picture frame to help roll out of there which will leave you on the lowest roof as well

you don't need the feather for the achievement. just roll in the 2 spots that i did, ( stairs on the bridge, and the small sign) and you'll complete this one with ease

You will need to use a Light Feather to roll over the barrier to the left of the Tower door. From there, you need to continue carefully down the roofs without falling until you reach a hidden balcony and door. Head inside to find the Staff Only area.