For context, Ghouls in my world function a little like zombies; whenever someone is killed by them, they turn in a Ghoul. A couple sessions ago, the party I am DMing went exploring in a crypt and unknowingly unleashed a few Ghouls into the wild. After exploring the crypt, they returned to the city where they got the quest from and then went out travelling. I had planned on having the Ghouls invade the city while the party was gone and decimate it completely, however, the party decided to head back to the city suddenly.

It has been to quick for the Ghouls to have attacked the city and moved on, so now I plan on having them stumble into the city in midst of the ghoul invasion. However, the party is currently level 6 and Ghouls/Ghasts can't really pose a threat to them. I'm having trouble thinking of ways to make this scenario harder/more interesting. What kinds of encounters can I throw in to make this more interesting?