Atop the World Tree is the seventh main level in the game, after The World Tree level.
It's a fairly straightforward level and a fairly simple achievement. You have to kill Griffins in nests to make leaf platforms activate. In one of the Griffin's nests is a bottle of wine. Pick it up after killing the griffin and then find the area that's full of spiky man-eating plants in the same level. Use the bottle of wine on one of the plants and it closes up and shrinks, allowing you to walk on its tile. Simply use the wine on all of the plants in this area so that they close up and shrink and the achievement will pop.