There's already a system in place for distributing military power--fleets. If you want to defend a border world, move in your ships. As an important bonus, you'd be able to see which systems are defended by fleets from the galaxy screen, without having to drill down to the system or planet screens.
Thats basically an argument for not having system/planetary defences at all. Which is a fair enough argument.
My point (i think...) is that *if* you are going to have planetary defences in the game, then they should be:
(1) Powerful enough on a well defended planet to make a significant difference to the outcome of a battle
(2) Not so powerful that they make assault an impossibility. They should never be able to stand up to a large (for that stage of the game) attacking fleet without some defensive ships to help.
(3) Expensive enough that a player would have to make significant sacrifices elsewhere if they wanted to have a large proportion of their worlds very well defended.
(4) The above 3 statements should remain roughly true throughout the game from the early stages (where fleets are tiny) through to the late stages with massive armadas.
IMO the easiest way to achieve this is to make planetary defences broadly behave in the same way as fleets, i.e. you can specify how many weapons of each type you want. This way they are always able to keep up with fleets.
Its all a matter of trying to balance offensive and defensive capabilities so that people cant be overrrun too easily... but also cant turtle to the point where every game becomes a stalemate. System defences *can* be a vital component in achieving this balance, depending on the game mechanics (although with starlane only travel defense comes down to holding chokepoints thus planetary defences to hold your 2nd line planets arent as important as your 1st line defences cant be bypassed as easily).
Edit : This is probably getting a bit off-topic
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