Just been reading the US account which gives a little more information about the attack, a little about what they did and how.
One point made was that some people had thought the Tongan guards may have been a little slack on their duties but the report also says that the normal procedure was that the tower was manned by 4 men, two upstairs carrying out their duties and two down on ground level, regarded there are off watch, sleeping, cooking etc until their spell upstairs. People may have seen the downstairs lads and mentally formed the wrong idea.
Another point mentioned was that two towers overlooked the area of the fence incursion but one would have been shielded by slopes and dips. only 2% of light available also made it hard to see.
Finally I noted that the towers were not build to see towards the airfield, only to their fronts and sides. I wonder if that caused any problems in the defence of the Harrier apron.