Around the outside ring there was a ditch that was filled with sharpened stakes. Your only entrance would have been through those notches in the big dirt mound ring. Around the top of the dirt mound was a road. It would have been difficult to gain entrance if you were an invading army. You also didn't have much of chance if you were in the local populace and you tried to overthrow those who were your overlords. Yes, Christianity was forcibly introduced at that time. Many liked their old gods and didn't want the new one. I guess I can understand that.
If you look at it from above you of course notice a general cross shape. It also crossed my mind that the four openings could also represent the four cardinal directions. Each of those were ruled by gods in Norse mythology. Therefore Christianity might be more easily accepted by the local Danes was what I was thinking. I may be a bit off there but I would like to see further studies done on the concept.