Trojan horse (1981)

Simple but effective game, you control the castle to the right to which soldiers and citizens run towards from left to right. The soldiers are recognizable by the helmets and the spears they wear. When a citizen arrives at your gate you'll have to open the bridge with the right button, or he will fall into the moat, when a soldier arrives simply let him fall...

At random times the Trojan horse appears causing the soldiers/citizens that walk underneath it to be invisible, they appear again when they reach the right side of the horse, but some of them will appear on the extended neck of the horse if his head is raised, which leads to the tower of your castle. When a soldier tries to enter your castle you shoot him with the left button when a citizen tries to enter you simply let him pass...
I rather enjoy this game, which is somewhat original in the sense that you control a castle instead of a character running from left to right avoiding obstacles..