var N = 30;

var a = 30;



function setup() {

  createCanvas(N * a, N * a);

  background(0, 0, 255);

  stroke(250, 250, 150);

  noFill();

  rectMode(CENTER);



  for (var n = 0; n <= N; n++) {



    if (n < N * 0.5) {

      rect(N * a * 0.5, N * a * 0.5, N * a - n * a, N * a - n * a);

    }



    line(n * a, 0, n * a * 0.5 + N * a * 0.25, N * a * 0.25);

    line(n * a, N * a, n * a * 0.5 + N * a * 0.25, N * a * 0.75);

    line(0, n * a, N * a * 0.25, n * a * 0.5 + N * a * 0.25);

    line(N * a, n * a, N * a * 0.75, n * a * 0.5 + N * a * 0.25);



    line(n * a * 0.5 + N * a * 0.25, N * a * 0.25, n * a * 0.5 + N * a * 0.25, N * a * 0.75);

    line(N * a * 0.25, n * a * 0.5 + N * a * 0.25, N * a * 0.75, n * a * 0.5 + N * a * 0.25);



  }



}