I'll show you five easy ways to say the same thing. 'Could not but let in' means the guards had to allow the cat to enter because they were surprised.
- The surprised guards had to let the amazing cat wearing boots and carrying a bag come in.
- The guards were so surprised that they let the wonderful cat in boots with a bag enter.
- Because the guards were amazed, they opened the door for the marvelous cat in boots who had a bag.
- The guards couldn't help it and allowed the marvelous booted cat carrying a bag to go inside.
- The astonished guards let the amazing cat in boots, which was holding a bag, come inside.