If you play as GDI, the enemy will attack your Weapons factory and your AGP from the entrance, the weapons factory is within sight. So you should mine the entrance of the AGP, be a hotwire (because only a hotwire can repaire an AGT while a flame tank is sizzeling it. And also mine the tunnels (only the second one to save on mines) so that they don`t take out your power plant while the other attack is going on.If you play as Nod, the GDI can attack your Tiberium Refinery, your Airstrip and one of your turrets without getting close to your obelisk of light. This is not good. So alway`s have an technitian (sorry about spelling) to reapair them. A good plan is to alway`s mine the second tunnel, and have some mines in front of the Obelisk, cause if they get that, game over! And the only real way to take it out is by beacon or mines inside. So protect it against all those attacks.
Sniper`s are great against MRLS`s and Mobile Artilaries! Also good for buggies and Hum-vee`s, but not as much! And even if those do make it near your base, their toast anyway!
Enjoy, and happy defending!
Peace