The climate you want is Estonia
Here is the link to Tallinn, Estonia's climate chart on Wikipedia.
The weather here meets your requirements, more or less. Pro-rating the months from 18 down to 12, you want 6 months of winter, and 2 each of spring, summer, and fall.
Tallinn's summer (Jul and Aug) are the only two months with average daily temps over 15 C. The spring (May, Jun) are the only other months that have more than 7 hours of sunshine a day, on average. And then the fall are the only other months where average temps are above 5 C. The rest of the year is winter. April doesn't quite fit this scheme, since the average low is above freezing and it is relatively sunny, but the other 5 months of winter are pretty wintery. There are 87 days of snowfall; about half the days of the 6 winter months.
The key to Estonia is that it has a maritime climate that keeps the maximum lows from being too extreme. The February average low is a balmy -7C. There are several cities that have much hotter summers, yet still have a colder winter. Examples would be Chicago (-9C in January) and Beijing (-8C in January). This means that in Estonia you can grow temperate tree and bush crops to help diversity the diet.
Agricultural production of the Baltic
To figure out what kind of civilization you would have, you have to look at what sort of agriculture they have. You can look up the agricultural output of Estonia, and similar countries like Sweden, Finland, and Latvia. Here is a detailed break down of crop production challenges in those climes.
For grains and high calorie products, the Baltics states were a traditional grain-growing region supplying food to the urbanized parts of northern Germany, the Low Countries and England. In more modern times, there is much potato and sugar beet production.
The south Baltic coast is a big center for apple production, although the summers there are a little warmer than in Estonia. Plums and currants are other fruits that can handle the cold well. Leaf vegetables do well in northern climes, like lettuce and especially brassicas like broccoli, turnips, cabbage, and (eww) brussel sprouts. Onions can also mature in the short growing season.
For fiber crops, flax used to be grown in great abundance. Flax growing is much reduced worldwide, as cotton replaced flax, but linen was popular in medieval Europe and would be in your world. Wool of course, is also popular in cold climate. Oilcrops are rare; butter would be the primary fat product. Canola oil, or rapeseed, was not grown widely in ancient times. It is a product of China, but in the last century has become the primary vegetable oil produced in the Baltic.
As far as animals go, cattle, sheep, and goats all survive well in such cold climates. Some of the best growing crops in such climates are fodder crops, like clover and grasses. This can be seen in the US, where the places with the coolest summers (Wisconsin, New England) are usually dairy producing states as opposed to grains, corn, or soy that thrive with warmer summers. Your land will probably be animal husbandry oriented, with lots of shepards and mixed agriculture/husbandry.
Conclusion
There is plenty of agricultural produce available to support large cities in this type of environment. I would imagine the warmer bits would have more fruit trees and grow grain in the longer summers, while in the colder climes, grass and clover would be grown and baled as hay for use with sheep and cattle.
