What are two reasons why Northern Europe was frequently invaded?

There are a number of reasons why Northern Europe was frequently invaded, but two possible reasons are:

- Proximity: In the past, the region was less developed compared to other parts of the world, and that made it attractive for invaders. Due to its closeness to the sea and its easy accessibility, people had a high chance of success when invading.

- Lack of a central authority: Historically, Northern Europe has been populated by relatively independent kingdoms and duchies, rather than a large, centralized state.

This lack of unity made the region more vulnerable to invasions, as there were no large, standing armies to deter or repel invaders.