What are S-Weapons
Idle Zombies Guide
As well as the regular guns that you are able to buy and upgrade in Idle Zombies as part of your defense there is a second category along the bottom called S-Weapon. You can unlock new ones the first time you reach level 30, 50, 75, 100, 150 in each city.
These weapons are powerful against the hordes of zombie and provide a constant defensive barrier but activating them comes with a cost.
Special Weapons
Unlocking the S weapon will effectively act as a prestige event for the city that you buy it in. You will reset your weapon purchases and zombies levels back to the start, you will even reset your dog house, although any auto work purchases that you have made for a dog will be saved and restarted once you unlock them again.
The advantage to thigh though is that you will immediately be stronger and will be able to fly through the initial stages again with a much faster earning rate and if you have other cities unlocked you will have plenty of income to very quickly buy and upgrade weapons to let you idle your way through.
For your first S-Weapon in San Fransisco we would suggest that you wait until you have London up and running with a healthy income before committing, but once you have this it is worth doing.