How to “embed” a small numpy array into a predefined block of a large numpy array?

you can use multi dimension index:

import numpy as np

wall = np.zeros((10,10),dtype=np.int)
block = np.arange(1,7).reshape(2,3)

x = 2
y = 3
wall[x:x+block.shape[0], y:y+block.shape[1]] = block

the output is:

>>> wall
array([[0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 1, 2, 3, 0, 0, 0, 0],
       [0, 0, 0, 4, 5, 6, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]])