Last session in Felltower, the group found what's been dubbed the "Icy Gate." Mostly because it leads to a land of ice and snow, it's ice-bound, in a freezing room, and beyond big metal doors that are themselves frost-limned.
So you can see why it picked up the name.
If the PCs go beyond it, what is around for icy weather gear?
It's getting on Summer in Stericksburg, but it's been a cold, wet spring full of owlbear attacks, so it's reasonable to expect you can still go right out and get some cold-weather gear.
Sleeping Furs (p. 23) are required for camping successfully.
Tents (p. 23) are also a great idea, even if Weather Dome is available.
Arctic Clothing (p. 16) This is a must-have. The old "Dryst will use Create Object to make us some!" is a risky choice. It beats plain-old Winter Clothing (p. 16) with a stick; it might be overdoing it but it might not be.
Camouflage Clothing (p. 16) is also a good choice.
Yurts (p. 18) would be a solid upgrade from tents, but they're heavy and special-order.
Skates, Skis, and Snowshoes (p. 18) are worth looking at. Snowshoes would probably be the best choice for the area just past the gate.
Polar Beer would be a an interesting choice, to reduce damage from friendly-fire flame attacks. However it's not commonly sold in Stericksburg. So that's two strikes right there.
Beyond these sources, magical support would be very important - Resist Cold, Warmth, fire spells (for offense and fire-starting), Water Vision, and Weather Dome can be delve makers or delve breakers.