Add contour lines on top of a density plot

Hi, I generated a colormap from a matrix N (figure below) :

The highest desnity is at the centre..indicated by color red and the least in the periphery (blue area). I want to add contour lines (circles) in this plot to indicate percentie data i.e I want to show a circle which encloses 25% data, a cirle which encloses 75% data and so on.

Such a representation would help me compare multiple such figures.

Note- I do not wish to make contour plots using contourf

Thanks in advance!!