Right and wrong. Movement and such can be done server-side, mainly. But visibility pretty much has to be done client-side. Which means that people can hack th

Right and wrong. Movement and such can be done server-side, mainly. But visibility pretty much has to be done client-side. Which means that people can hack the client to see through walls. You can do culling server-side to an extent, but not entirely - because otherwise someone with a high latency coming around a corner will have no chance against someone with low latency on the other side. Also, servers pretty much have to (well, not really. But otherwise games become unplayable to those with higher latencies, as where you see other players is not where you need to fire to hit them) allow you to hit someone that was, according to your client game tick, in target when you fired. Which means that the client has to tell the server what game "tick" the client fired on. Which allows for cheating if the client deliberately tells the server that it fired/started to move on an earlier tick than it actually did.