The Bavarian portal was originally deep in the woods between twin trees that grew between twin medieval castles. When Mother Goose sends the Grande Armée through, she asks King Ludwig II to build a castle over that exact spot so the soldiers won't find their way out. 
Conner, Bree and Emmerich go into the castle to find the entry to the portal, which turns out to be a painting of a magical garden; "a majestic forest with trees, flowers, squirrells, deer, and boulders."
The portal can only be activated by a person of magical blood playing 8 tunes on a magic panpipe. Mother Goose gives Wilhelm Grimm a few drops of her blood so he can open the portal for the French soldiers. Conner is the one who opens the portal in A Grimm Warning; he is part-fairy. They only want to test if the portal works, but get sucked in by its force. It is a rough ride; they feel very discombulated when they arrive.
The portal takes 200 years to travel through for persons of non-magical blood, like the French soldiers. Conner, Bree and Emmerich take roughly a few weeks, implying that they all have magic in the blood (not just Conner).
In Beyond the Kingdoms, Morina tells the other witches how she managed to use the portal to go to the Otherworld to kidnap Emmerich, but adds that the white magic the late Fairy Godmother used to create it weakened and exhausted her, and claims the other witches wouldn't have the strength to use it.
The Otherworld-side is inside Neuschwanstein castle in Bavaria, Germany, in a room called the Singer's Hall. On the Land of Stories side, the portal leads to a site near Pinocchio Prison in the Eastern Kingdom.