I find that WP with several PD weapons and missiles can stalemate or win against any ship that tries to duck in and out of weapon range. Especially if you have missiles with a range of 10 or more as no direct fire weapons can fire from that range.
With regard to transporting them via colony ships, the smallest WP is 200kt in size so colony ships can't move those puppies until late in the game. The transports you normally load with population can. However, even the smallest world can build a cheap WP in one turn, so there's no real reason to transport them around.
A more advanced question is how do they effect damage distribution? For example, a planet has a facility, 10M people, and a WP. It gets his with a missile for 100 points of damage. Do the people die, the facility takes 100 damage, and the WP takes 100? Does it get eveny distributed? Does it hit first WP, then Facil, then population? Or, as I think, does it get randomized like components on a ship?
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