Naval invasions are quite common. I'm playing as Makedonia, and due ot its central position, it's naval invasion every ~20 turns on average. KB, Sarmatians, Ptolemies, Pontos, SPQR, Carthage, Massylia, and recently even Aureakoi invaded me by sea, most of these multiple times.
I keep 5 units+governor in every settlement and several armies around borders, but it isn't really working as deterrent.
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