How are you handling NAT traversal?

handling latency to keep a coherent experience in a real-time game is an unsolvable problem where every single mechanic requires a planned out scheme and companies go so far as buying data centers in optimized locations to help out with the illusion

that's probably what you mean by highest level of complexity

but even just a regular turn-based game there's a lot of questions you need to answer and decisions to make regarding latency, packet loss, and disconnects that all have to be solved in a coherent and consistent way