The course is fairly modern and when it was built there were very strict instructions about what could or could not be done in the way of earth-moving and tree removal. The trees (mostly scrubby bushes) line the higher part of the dunes which form the secondary sea-wall which protects the farmland beyond, which lies below sea level. I think the other thing (in addition to the odd quirky hole to which Mike refers) that keeps Noordwijk just below the highest level is that the short holes are fairly ordinary.
Those who have not heard of the classic Dutch courses should look up Kennemer (even nearer to Amsterdam), De Pan (Utrecht), Eindhoven, Hilversum and Royal Hague (see the excellent pics posted earlier in the year).