Making sure that movement is validated on the server really is a reasonable requirement. There are two main issues: 1) You typically want to let player's mach

Making sure that movement is validated on the server really is a reasonable requirement. There are two main issues: 1) You typically want to let player's machines have a little knowledge the player doesn't, because if they peak around a wall, they should see an enemy, without having to wait for a round-trip before getting the information of the enemy they can see. Games often give out too much of this kind of information, but that's because it is computationally hard to figure out if a player can "almost" see an enemy. 2) Aimbots are basically impossible to check for on the server -- you can play a cat+mouse game (and games do) of seeing if players are consistently too accurate (or more likely, consistently too mechanical), but it's very hard, and player-side anti-cheat is part of trying to control this.