OK, looks like the First Order has another ginormous spherical planet destroying spaceship. Being an engineer, I get that it makes sense for the bad guys to recycle this design:
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The Return of the Jedi Death Star supposedly had the bugs of the previous one worked out (according to the novelization) and probably would have been nearly unstoppable if the Rebels hadn't been able to destroy it while it was still under construction. So the bad guys have a good off the shelf design to draw on and no real reason not to start with the last Death Star blueprints.
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A sphere allows the most interior room for a given surface area - and hence, the most interior room for a given weight of armor.
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The vulnerable reactor at the center has about the same distance to any point on the surface, so you've got to punch through the same amount of spaceship to hit the reactor from any direction.
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It's probably easier and more efficient to generate a spherical force field to protect it than other shapes.
OK, so all the bad guys have to do is build their spherical death machine and make sure it's protected against flying small spaceships in through its defenses, since that's how the Rebels took out the last two Death Stars, and also how Anakin took out the droid control ship, right?
So, really, shouldn't the good guys need to look for a better way to blow up the latest knock off of the Death Star? Sure, a space battle against a bunch of capital ships may be a rather boring slugfest of attrition. So what options would offer more drama? Maybe have a commando raid sabotage the reactor and rig it to blow? Hijack a Star Destroyer, pack it full of something explosive, set it on a collision course, and hope you can make it to the life pods in time to bail out? Rig something to come out of hyperdrive right in the middle of the spaceship?
Surely both sides ought to have learned something from the last time...