Stu:
One thing to consider is that the swamp may have moved. So, just because the place that Sartar built was a port when he built it does not mean that it is now. The swamp may have receded (in fact, that could be a side effect of the road's magic), requiring a new settlement for port traffic. That might be why the road ends in a ruin that's not currently a port.
Also, it would certainly be to the ducks' advantahe during the lunar occupation for the lunar army to think that an empty ruin is the ducks' only settlement. It would both deprive them of any target and also signal to the city-loving northerners that the ducks are no real threat. It's duckish reverse psychology at work.
Chris
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