City Guard signet ring, is given to Geralt by Vincent Meis in Chapter II only after the witcher finds irrefutable proof that the captain is not involved with Salamandra. It allows entry into one of two semi-restricted areas within the Temple Quarter: the dike and St. Lebioda's Hospital. Although this ring can not be bought initially, once acquired, it can be sold and then repurchased.
Depending on how the plot has unfolded, Geralt obtains this ring for one of two purposes, either to gain access to the Swamp via the dike or to access St. Lebioda's Hospital.
Access to the Hospital
If Geralt teams up with Siegfried to fight the cockatrice in Prison Break, then the hospital is guarded by a city guard who requires either the signet ring or a bribe of ~50 (sometimes less is fine but 50 is safe) before he will let Geralt enter. However, Siegfried waits by the gates to the dike until Geralt arrives and vouches for him when he tries to go to the dike, securing passage to the swamp.
Access to the Dike
If Geralt worked alone in the sewers, then the signet ring is helpful to gain access to the dike. Siegfried will vouch for Geralt at the hospital, which allows him to enter without the ring. The guards at the dike, however, need to be presented with the signet ring, or bribed with 55 to let Geralt pass.
- The city guard signet ring is only useful in the game if Geralt does not resort to bribery.