hello i need help to get inside the fortress and a sort of rubber wall is blocking the way in the swamp
The rubber walls in the swamp can be cut down with a tool called "Flimsy Shears". You can find the schematics for this tool in the swamplands, in the serpentfolk's lair.
You can either be defeated by them and get sent there... or you can follow the path through the swamp, and use ranged projectiles to knock down the upright bridges.