Unless they have moved the goalposts, it's worked out on distance usually, although the competition players will verify that.
Incidentally, Graeme McDowell who hinted at the course being too easy.................sank a 15 foot putt for a nine at one of the holes, aye, easy right enuff Graeme.
They are all easy if you hit the fairway and green in regulation, therein lies many an amateurs problem.
Taken from wikipedia which may be near enough:-
The key factor for classifying the par of a hole is the distance from the tee to the green. A typical par-3 hole is less than 250 yards (225 m) in length, with a par-4 hole ranging between 251 and 475 yards (225?434 m), and a par-5 hole being longer than 475 yards (435 m). The rare par-6s can stretch well over 650 yards (595 m).
You normally find that a par five hole would have a smaller green as you would be expected to be near the green with your second shot, thus making the third approach to the green a short pitch (hopefully)
The issues at Castle Stuart as I understand it from a scoring perspective is that the fairways were too wide at the driving distance and too generous, ie, the rough was not very heavy and the bunkers could have been more intrusive. However, take yer normal Jack the Lad round here and he will probably return scores in the nineties.
These guys are experts, in fact Luke Donald is the current world number 1 and he made it look easy because he was at the top of his game.