After sketching various specific Japanese locations, I did a quick painting of the "city" which would include them. The castle would anchor all of them as a background element.. in theory. I chose the daytime setting. Ninjas sneaking around under cover of night would be too predictable - no, my ninjas will be doing their laundry and taking their morning jogs under light of day!
Next was character development. Two characters: either a protagonist/antagonist pair, or hero/sidekick. I determined to create a ninja family, with an awkward Dad (guilty of all the tropes; bad jokes, sweat pants, fishing, newspapers, etc) and a responsible daughter type.
#3 was taken to finish first: