Planetary Defense

This is an old question, but it's worth having an answer out there.  Unfortunately, the answer is no.  The 25% bonus isn't coded and it's a worthless structure.

It's something I fixed in my v1.1 mod and remains fixed (obviously) in v2.

So, I also guess that the space cannon the Arceans have is also useless defence wise.

But does the Cathedrals of Valor work?

The Space Cannon does work, the 50% bonus is inserted where it should be.  Planetary Defense doesn't work simply because "they" neglected to put the 25% bonus in the XML.

The Cathedral of Valor works with respect to the 33% planetary defense bonus.  The 2% soldiering bonus doesn't work because that type of bonus can only be given by a Galactic Achievement, Super Project, or Trade Good.  I addressed the issue by making it a Super Project with a much higher soldiering bonus (20%) and also morale and resistance bonuses.

Curiously, I have almost never seen the AI build Planetary Defense structures in any version of the game, even though they regularly research it (of course, the tech provides a soldiering bonus just by itself). By way of contrast, as soon as I fixed the issue that was preventing the Arceans from researching Space Cannons, they make regular use of them, as do other civs that get traded the tech.

At a guess I would say that the AI notices that the building (Planetary Defences) doesn't confer any bonuses whatsoever, and decides to build something else.

I have seen the AI make use of Planetary Defense, though not often and not entirely prudently (smack in the middle of the empire as opposed to a contested border planet.)

I can certainly say that the Arceans use the Space Cannon and that the bonus works, because the difference in casualty rates is very noticeable after it gets built.

Plus it actually shows the bonus under the improvement text.