You can't actually circle. So basically it needs to be large enough that you can come down, brake in a straight line, reach safe speeds, 2x safety margin, and then slow down and enter the inner airport.
Its basically a size issue: And airplanes quickly burn trough 1-2km of runway, or more.
So i assume 15-18 degrees of the runway is fine, to get a nice "straight line" to work with.
Now, because Pi is Pi, if the circle is around tower(as illustration), and there is 1km to the tower, you have a circle thats 3,14km long. Where 15-18 degrees is 200 meters, which isn't long enough to land or break on.
Now, to get 1km of "brake area" before you risk turning: You would quickly need there to be 5km from the tower to the circle. At which point you could just have ignored the circle and just built the runway in regular fashion with those 5km needed to get the circle that large.
It basically can't work if you need more than 200-300m to brake, because it will take up too much space.