With the Helmet Key, you can now access the mansion's basement. Head back to the Main Hall. On the first floor, use the Helmet Key on the door located to the left of the Dining Room entrance. This leads to a short corridor.
Follow the corridor. You'll pass a door on your right (often a storage room with some ammo). Continue to the end of the corridor and go down the stairs. You are now in the Mansion Basement. This area is significantly darker and often contains more dangerous enemies, such as Cerberus (dogs) and Hunters. Be prepared for combat and conserve your resources. Explore this area thoroughly, as it contains crucial items for progressing to the Guardhouse.