You mean every human was just born this way? Every person just has this field capable of withstanding 20 full force hits from a two-handed long sword?
The simple answer is that a lot of medieval weaponry wouldn't have been invented, and what weaponry was created would mostly be for the purpose of hunting animals not humans.
Humans would most likely have learned to use their energy shields against each other in fights rather than build weapons for that purpose, given that the field as described would have been the hardest, and most durable, thing known to humans at that time.
For example: you state that the "the armor could only become weaker than another point by a user deliberately shifting energy around themselves", which means that other areas would presumably be made stronger by doing this. The obvious question is: to what extent? Could I condense all of the power of my energy field down to a cylinder 1cm or so in diameter that projected 30-40cm out from my right hand? If so then I could presumably punch that through someone else's energy shield and their body.
Come to think of it, you describe this as a field, which means that it is not constrained by 3 dimensional physics. I could therefore theoretically project the field outward as a 2 dimensional plane that had width and length, but no thickness. 477
That plane would be able to penetrate other fields because it would have no actual surface area on the leading edge. It also means that it would pass through the person inside the field without damaging them - again 2 dimensions, so no penetration. So what would be the use of this? Well the field wouldn't harm the person as it passed through them, but it would create a barrier that would not allow things like oxygen or blood to pass through it.
Beyond that most of the weaponry created would probably be designed for distance attacks using projectiles with a high mass since the field allows inertia to be transferred to the user.
I also expect there would have been a lot more use of fire as a weapon since the field as you describe it seems to resist penetration but otherwise still allow energy to pass through it. 477