Anyone who feels like Zurvan, as shown in the screenshots in the OP, is too open or would be too easy to attack really should take a closer look. There are thee factors that make this base very defensible.
First: There two good sized walls with towers at each end protecting the main facility from two of the more open approaches.
Second: The walls and towers have 2 levels of turrents on them, Anti-vehicle on top and anti-infantry on the bottom(I At least that's how I recall it from TB's streams with Higby)
Third: Yes, the far side of the base is open but terrain has clearly been used to create natural bottlenecks. Rock formations stand in the way of pretty much all the roads leading into main facility.
It's not as easy as it looks at first glance; it's just going to take a bit more creative of a defense than that of a castle-type base. You put a platoon of guys on the walls manning the turrets with a healthy mix of snipers, AT and AA on top, some tanks/mines/AT turrets just inside the natural bottle necks on the (camera)north/north-east approaches and any ground assault will have a hard time getting through.
Of course this all depends on how well each faction is communicating and organizing. Having good commanders to over see this kind of stuff will be key, and more important than the base layout it self in most cases. The faction that works together, will win together. Mostly.
Ultimately though, the bases are going to be varied in terms of lay-out, amount of walls/buildings/natural barriers and just the pure difficulty of attacking/ease of defense. They've said it at least a few times and I feel like I've heard it quite a bit more than that. This is a good thing in my opinion. Variety is important to keep the game play fresh and those who like castle sieges can do that, those who prefer more open field combat can do that too.
Me? I'll be in a Mosquito, so much of this is null