The ongoing excavations at the early-medival Borgsumburg fortification aim to understand the construction of the northern and southern gate. Whereas the southern gate, pointing to the moraine area of the island, is known since long, the northern gate was detected only recently. If you follow the path to the North through it down along a diagonal ramp, you end up in the marsh in the area of a silted-up basin, which was likely used as a natural harbour, once connected with the North Sea by tidal creeks.

In the areas of the former gates, rows of stones point out the lay-out of the entrances with curved sides and, especially in the southern gate, a very narrow passage at times.

The moat outside of the rampart was investigated by a trench. It was about ten metres wide and 1.5. metres deep.  Presumably it was filled with water at times. The inner side of the moat was buried by sods of the rampart, which means that the front of the rampart must have been even steeper than today.