Fortresses are a joke in the current version

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I'd argue that Sins of a Solar Empire has shown pretty well that static defence buildings can work in a 4x-ish game.

However, when it comes to Stellaris it's a bit more tricky to adjust the power level of defences, given that tech has a pretty massive impact on ship quality and that Stallaris doesn't have as big of a diminishing returns for having a giant fleet size.

 

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I think what made the star-bases of SoaSE so great is that they could be upgraded to fit a variety of different strategies. Yes, you could build them for the purpose of buying your fleet time to reinforce the system, but they could also be built to do other things. You could use them to progressively wear [...]

Don't mind me, just putting down 4 Advent Bases with T3 culture upgrades next to the main star.