Grippli word for the place is unpronounceable for humans, so most of drownlanders simply call it Frogport. If you don't know the right people you'll never find it, my friend, since it's always moving, unerringly navigating the labyrinth of Drownlands' numerous rivers, never stopping. Its location a secret, its presence an opportunity or a threat. How is it moving, you ask? Some say it's built on a shell of a giant turtle. Others whisper about blood sacrifice, human visitors going missing and dark and terrible magics. But no one really knows, my friend. No one, but the grippli.
So in Scotland and Ireland some ancient peoples lived in crannogs, fortified dwellings on artificial islands. They looked like this:
Grippli's fortress looks like that, but it's much bigger, storied and partially hidden in a dense thicket. It is also always visibly moving, sometimes even upstream. Grippli divulge the current location of the fortress only to those considered friends or at least trusted associates. Outsiders are admitted only if an adult grippli will vouch for them. To get in you have to pay a grippli ferryman to take you there in a small reed boat or (more costly) a raft. Grippli themselves mostly just swim there, unless they have some sort of delicate cargo that they don't want to drench.
Why visit Frogport? Grippli are the masters of alchemy. They brew the most unusual and powerful potions, but are especially famous for their poisons – said to be deadliest in the world. Most of the time the grippli alchemists need to ingest some kind of rare ingredient to synthesize the needed chemical compound through their own bodies like some tree frogs. In exchange for their their services they accept metalwork (especially iron or – even better – steel weapons), textiles, glass, rare minerals and gems, ice blocks from the north and exotic plants and fruit from the south. Coins have little value to them (prices are 500% normal if paid in coin).
The biggest secret of the Fortress, one which the grippli guard with their lives, is how is it really moving? It's built on top of Frog Goddess the Beloved All-Mother Always Egg-Laying. She's gargantuan and older than dragons. She leaves behind her an endless trail of eggs. Only some of them hatch and only some of the tadpoles survive to adulthood, but those who do become Her Saints, her miracle doers, her secret priests and prophets. To outsiders they camouflage as druids. Other grippli treat them with utmost respect and discreet reverence. The highest mysterium of their religion, known only to the select few of the highest priesthood, is the identity of the Saints' father.