Covering The Sewers
While it did not include one fool-proof plan, it still had notes inside about how each and every able person in Wecha had a role to play. They had been prepared for the rats to play dirty. While not as numerous as Moloch fabulous playbook against the Demon Horde, Gold still had formulated dozens of what if scenarios which he submitted to the System based on the current information they had on them.
Naturally, one of them had always been an underground invasion. After all, they know those rats were capable diggers long before they got access to Weslie's steam technology.
Thus the sewers had been rigged with explosives, all meticulously covered and camouflaged with mud, dirt and smelly garbage. The foul-smelling job had not prevented the citizens from earning extra coin which the System had an abundance of in the weeks before this ambush.