If hostile creatures are spanning within a workbench radius inside your walled base, something has gone wrong with the copy of the game on your device.

Validate your game files. You have no mods now, did you ever have any? If so, there may be a residual mod file that was missed in the uninstall process. You might have to uninstall Valheim, delete the folder manually, and then reinstall Valheim.

A word about the workbench radius. It is the radius of a hemisphere not a cylinder. That means that while the base of your wall may be in the workbench radius the top of the wall may not. For example, if you have a 4 meter high circular earth wall whose outside edge is just within the workbench radius, the top surface of the wall will be outside the workbench radius. Mobs can spawn atop the wall and slide or jump down to the base interior.