Primarily defend the greens by their surrounding hazards
Although it is fun once in a while to play extremely undulating greens, regularly four putting on roller coaster greens gets tedious pretty quickly for most mortals. By making playing to certain parts of the green hard, it is possible to make the greens challenging for the better players, whilst still making the greens playable for the rest.
At IVGD we generally take it easy with our green undulations. Many would probably describe our greens as only mildly undulating (although some might say we did more than that at our latest course De Swinkelsche). Instead we provide many short grass runoff areas around the greens to make things interesting.
“The majority of players desire to hole out in two putts on each green if they are putting well. They desire to experience some little difficulty in doing so, because otherwise they would derive no pleasure from success. Therefore a perfectly flat green would not satisfy them. On the other hand, when they have avoided the bunkers of the fairway, when they have eluded the traps which guard the entrance, and have played their ball on to the putting green, they do not like to find it is lying in a severe form of hazard"
Harry S. Colt “Some Essays on Golf-course Architecture” 1920