Royal Guards, or Kings, even Thegns would be a problem. But, that's all part of how the game is suppose to be. Once the Vikings try to start pushing inland is where they are suppose to start having a much harder time with it. The Christians needed to protect key points and just write off the rest. They can't hope to protect everything. I was surprised at how many city improvements were still in cities when I took them. I expected the cities to have been stripped.