In New Lands
For the winter in Kingdom: New Lands.
Surviving winter to a point is certainly possible, if difficult. The Steam achievement denoting the survival of Day C is certainly motivation for many.
Focus should be put into farming early on, as it is the most consistent money source. A fair amount of hunters and large tracts of open plains makes for hunters hauling in purse loads of coin each day. The Kingdom should exploit every source of income possible and stockpile everything with the Banker. The Keep can hold unlimited coins, which on top of any merchant shipments (if he exists that land) can bring you well into winter.
A strong defense against the Greed should also be set up; however, if the Kingdom is lacking funds and the strongest, outermost wall falls, the rest of the Kingdom will inevitably follow within a few nights. If the Kingdom has stockpiled coin in the summertime, it should be able to recover from several strong attacks before dying. It also helps, if possible, to keep the Greed's mother portal in the cliffs incapacitated after all the other portals are destroyed.
In Two Crowns
For the winter in Kingdom Two Crowns.
For newer Kingdom Two Crown players here is an advice to survive winter which starts on day 49—usually when you see the L roman numeral is a sign that winter is near.
If you don't want to reset your days for your personal high score/most days survived I'd recommend going back to the first or second islands where there are less portals and the merchant is present so you can sustain the day. Invest a lot on these islands since they have the minimal number of portals. Always put your coins in the bank since it'll passively generate investment for you. I'd suggest to invest in pikemen in Europe and in ninjas in Shogun: they still fish throughout winter. Even though it's not much, it's better than nothing and you can survive with it. Ninjas though die much easier and are often running around most of the day. If you pay one coin to berry bushes and you have any farmers they go and harvest the berries which grow back daily. Winter ends after sixteen days.