He has magic users working for him; right?
The Imperial Fireworks. Some sort of bronze and stone monstrosity that looks like a pipe organ mated with half of the sculptures in the art museum. Every night just after dark, it fills the sky with a shower of light and sound, a fireworks show. On the night of the emperor's birthday (and maybe other special nights), it showers the sky all night long (keeping everyone in the city awake, of course). No matter what, the thing cannot be turned off.
If the emperor is a lunatic, have it be real fire where occassional sparks fall back and start fires in the city. If the wizards are smarter, then it's just illusionary light and sound.