Enemy moving trough border

import turtle
import os

#setup the screen

wn = turtle.Screen()
wn.bgcolor('black')
wn.title('Space Invaders')




#Draw border
border_pen = turtle.Turtle()
border_pen.speed(0)
border_pen.color('white')
border_pen.penup()
border_pen.setposition(-300,-300)
border_pen.pendown()
border_pen.hideturtle()
border_pen.pensize(3)
for side in range(4):
    border_pen.fd(600)
    border_pen.lt(90)

#create the player turtle

player = turtle.Turtle()
player.penup()
player.color('blue')
player.shape('triangle')
player.speed(0)
player.setposition(0,-250)
player.setheading(90)

#Enemy
enemy = turtle.Turtle()
enemy.penup()
enemy.color('red')
enemy.shape('circle')
enemy.speed(0)
enemy.setposition(-200, 250)

enemyspeed = 2


#Movement
playerspeed = 20




def move_left():
    x = player.xcor()
    x -= playerspeed
    if x <-280:
        x = - 280
    player.setx(x)


def move_right():
    x= player.xcor()
    x +=playerspeed
    if x >280:
        x = 280
    player.setx(x)


#create keybinds
turtle.listen()
turtle.onkey(move_left, 'a')
turtle.onkey(move_right, 'd')

#main loop
while True:

    #move the enemy
    x = enemy.xcor()
    x += enemyspeed
    enemy.setx(x)

    #Enemy Reverse
    if enemy.xcor() > 280:
        enemyspeed *= 1

    if enemy.xcor() < -280:
        enemyspeed *= 1