:A=15
:N=5

REPEAT 3
 [
  REPEAT :N
   [
    FORWARD :A
    RIGHT 90
    FORWARD :A
    LEFT 90
   ]
  RIGHT 90
  REPEAT :N
   [
    FORWARD :A
    RIGHT 90
    FORWARD :A
    LEFT 90
   ]
  RIGHT 180
  FORWARD 2*:N*:A
  BACK 2*:A
  RIGHT 90
  :N=:N-2
 ]