The other answers have addressed the feasibly of building the wall, which is within the reach of an empire that has the will to do it. You can even scale it back significantly, particularly the thickness, and still have a daunting barrier for a 15th century army.
But what happens once it's built?
If it's purely symbolic, great! Done. But if it's for defensive purposes, a wall alone does not make you safe. If unmanned it is merely a speed bump for an attacking army.
An attacking army need not go over or even through the wall, but they can go under it. Undermining is a military siege tactic of digging a tunnel under a wall. Rather than using the very narrow tunnel to bring troops in, it's used to make a gap in the wall. Prior to explosives, typically a void is dug under the wall and then the tunnel is collapsed by removing or burning the supports. In later times the tunnel would be packed with explosives. This would require far less explosives than trying to blow through the wall from the outside as it collapses the foundation causing a section of the wall to fall into the resulting hole.
Such a large and circular wall, its location picked for its shape and not to match the terrain, would be vulnerable to easy undermining at many locations: rivers, swamps, sandy areas. In some cases water would naturally undermine the wall providing easy access and even eventual natural collapse.
Manning the wall
An undefended wall is a useless wall. The wall will have to be manned. In times of peace by lookouts. In times of war by significant forces able to repel an attack and prevent undermining.
If we assume a radius of 45 miles, your wall could have a circumference of about 283 miles. To put this in perspective, Hadrian's Wall, which was more of a border checkpoint than a defensive position, runs 73 miles. It had a gate with a milecastle and two turrets every mile, and a larger fort every 5 miles. The milecastles were garrisoned by static troops while the forts provided a mobile reserve of 500 to 1000 troops to respond to emergencies in the milecastles.
- 1 milecastle per mile
- 2 turrets per mile
- 1 fort with 500 to 1000 troops every 5 miles
Your wall is 4 times longer. If we assume the same peacetime manpower as Hadrian's Wall your wall needs...
- 283 milecastles
- 566 turrets
- 56 forts
- 72,450 mobile troops
Mobile troops would need horses, supplies, laborers, blacksmiths, supplies, and so on. The milecastles could be manned by local conscripts, and probably a few professional soldiers to keep them in line. The conscripts would live nearby to reduce supply problems.
Hadrian's Wall was worth the effort because its 73 mile length protected the whole of Roman England from attack from the north. Your wall protects a single city. Maintaining an army of 72,000 professional soldiers, a very significant army in the period, in peacetime to defend one city gets very expensive.
Because your fortification is so much more significant than Hadrian's Wall, you could probably reduce your manpower requirements and still have a viable defense, but it will still require a quite significant standing army simply to defend it. Otherwise it's a speed bump.