Random walls

To make the walls appear randomly, we'll first need to import the random module.

import random

Next, we'll choose a random number. The range is up to you, but I'm going with 0 to 100.

random.randint(0, 100)

...and we'll draw the wall only if the random number meets a certain condition, for example, if it is less than 50.

for x in range(16):
    for y in range(10):
        if random.randint(0, 100) < 50:
            wall = Actor('wall')
            wall.x = x * 50 + 25
            wall.y = y * 50 + 25 + 50
            walls.append(wall)

Since the wall pieces are drawn randomly, your screen may not look the same as mine.