An overlay lets you ‘draw’ over a map. To add an overlay to a MKMapView:

1. Start with an existing MKMapView or review the Displaying a Location recipe.

2. Declare class-level variables for the overlay and its renderer:

MKCircle circleOverlay;
MKCircleRenderer circleRenderer;

3. Implement MKMapView.OverlayRenderer to provide a renderer for the overlay:

mapView.OverlayRenderer = (m, o) => {
    if(circleRenderer == null) {
        circleRenderer = new MKCircleRenderer(o as MKCircle);
        circleRenderer.FillColor = UIColor.Purple;
        circleRenderer.Alpha = 0.5f;
    }
    return circleRenderer;
};

4. Create an overlay, in this case a circle positioned near the Pyramids of Giza, and add it to the map:

var coords = new CLLocationCoordinate2D(29.976111, 31.132778); //giza
circleOverlay = MKCircle.Circle (coords, 200);
mapView.AddOverlay (circleOverlay);

The OverlayRenderer method must be assigned to prior to adding the MKOverlay to the MKMapView. If this is not done, the overlay will not appear until the map is moved or scaled in some way.

The MilesToLatitudeDegrees and MilesToLongitudeDegrees helper methods can be found in the Displaying a Location recipe.