The error you receive is: Bummer: Did you create a new class named Square?

Here you have Polygon and Square reversed;

class Polygon : Square

should be

class Square : Polygon

After fixing that the next error you receive is: Bummer: Did you define your Square class as an inner class of the Polygon class? Be sure to define the Square class before or after the definition for the Polygon class.

This has to do with your brackets. Move the second to last closing bracket up so that it closes the Polygon class before the Square class. With these changes you'd have:

namespace Treehouse.CodeChallenges
{
    class Polygon
    {
        public readonly int NumSides;

        public Polygon(int numSides)
        {
            NumSides = numSides;
        }
    }

    class Square : Polygon
    {
        public readonly int SideLength;

        public Square(int sideLength) : base(4)
         {
              SideLength = sideLength;               
         }    
     }  
}