The retreat room is the whole strategy
Almost everything about a successful party, from your cat's point of view, is decided before the doorbell first rings. Choose the quietest room in the house — ideally the one she already sleeps in, furthest from the music and the front door — and set it up completely: her bed and a covered hiding spot, litter tray well away from food and water, a scratching post, and something that smells strongly of home, like her usual blanket. Move her in with a meal or a favourite game before guests arrive, while the house is still calm, so entering the room is her idea executed early rather than a rescue performed over rising noise.
Put a simple note on the door if it helps: cat resting, please do not open. It saves every guest improvising, and it saves you from discovering mid-evening that someone opened the door to say hello and let a frightened cat out into a hallway full of strangers' legs.
Brief the humans, because guests are the variable
Cats cope with noise better than with unpredictable people. The real risks at a party are a well-meaning guest pursuing her for a cuddle, children opening the retreat door, and someone feeding her rich party food or leaving drinks where she can reach them once she is let out later. A short, cheerful briefing as people arrive covers all three: the cat is off duty tonight, her room stays shut, and nothing from the buffet goes to her, however hopefully anyone imagines she is looking at it.
Assign one household member as the quiet checker who slips in once or twice during the evening to refresh water, check the tray, and sit with her briefly if she wants company. Everyone else's job is simply to keep the door closed and redirect curious guests. If your cat is deeply social and genuinely enjoys visitors, a short supervised appearance on her own terms, with an escape route open the whole time, is fine — but let her choose to leave the moment she has had enough, and never pass her from person to person.
Manage noise, doors, and the escape routes
Think about the house as she experiences it. Keep speakers and the busiest gathering spot at the opposite end from her room, close internal doors to create a sound buffer, and be strict about the front door, which will open far more often than usual and often stand open while people carry things in. A cat who does slip out of her retreat will usually bolt for the quietest dark corner or, worst case, straight through an open external door, so that door discipline matters even when she is supposedly secured.
Watch the small hazards a party scatters around: candles at tail height, dangling decorations and ribbons she may chew later, cleaning sprays used on spills, and plates of food left on low tables overnight. A ten-minute sweep before bed, once the guests have gone, removes most of what a curious cat released from her room at midnight would otherwise find.
The morning after is part of the plan
Let her out when the house is fully quiet again, not into the middle of the clear-up with furniture moving and bags of rubbish at the door. Offer her normal breakfast in her normal place, open her usual routes, and let her inspect the party rooms at her own pace, sniffing the evidence and re-marking her territory. Most cats repossess the house within the hour once it smells and sounds like itself.
If parties are regular in your home, keep the system identical each time — same room, same setup, same door rule — because predictability is what turns an ordeal into a routine she can sleep through. Writing the retreat setup and the door rule into KinsLe's Family Sharing before the event means partners, older children, and housemates all brief their own guests the same way instead of relying on memory.