An oft-cited fear is that compact densely-populated OA-Cities would make ideal terrorist targets

The aim of terrorist groups is to kill as many enemy targets as possible, so any place with a high population density will make a more tempting target for them than low-density suburbs or rural areas. 

OA-Cities will certainly be high on terrorism’s target list but, that said, their very compactness coupled with their controlled-access points – will make them very defensible and almost impossible to infiltrate compared with conventional OB-Cities which are so open to infiltration.  

None of the terrorists’ favourite mass-killing methods would work against OA-Cities

ALLAH AKBAR..!  Nice truck-ramming atrocity July 14, 2016

• Motor vehicles will have no means of access, so truck/car bombs, etc, or using vehicles to run-over scores of people would be impossible to stage.  

• A dual layer of airport-style security control – at the outer perimeter and then at the building entrance itself – will make it very difficult to smuggle in weapons or bomb-making materials.

• Any weapon or explosive device which did manage to evade these controls would be quite small and relatively ineffective in a large building.  
• In a worst-case scenario where a hijacked plane manages to score a direct hit, a pyramidal building’s innate structural stability would prevent it collapsing catastrophically like a sky-scraper.     

A pyramidal OA-City would NOT collapse into a heap of rubble like the twin towers.

• Unlike closely packed sky-scrapers, OA-City modules will have a “clear field of fire” and, depending on future political conditions, they COULD be protected by radar-controlled “Phalanx” Gatling guns.