The Campaigning Season Is a Real Thing

Vikings could attack during inter because boats still have to be powered by men, it's a lot bigger. This is also how you get sometimes some very large, for instance, roman armies. The logistics of step nomads are different from the logistics of agrarian armies. A small army, 20 thousand ish or less, could manoeuvre almost anywhere in europe this way. But as your armies get larger, where you can go is limited.