Base fucking guide
Defending, everyone knows what it means. But how does one do it good? Some say "Take a good unit and camp! That's the way." Wrong, there is much more to it then just that. camping and defending are totally different.
One of the key parts for victory is the integrity of the base, how many of your buildings have been destroyed, if you still have base defenses, etc. To mantain the base operating at full capacity it must be defended.
 Importent Notes
This topic isn't complete! If you feel something is missing, feel free to add it to this topic.
 Before Defending
As a wise general once said, "Know thy terrain and you shall win". This is true! In order for you to succesfully defend you need info about everything!
What buildings does each team still have? How is the terrain? Who is in my team? Who in the other team? Do we need to defend? This effects where you guard, depending on what map it is (Hourglass/Under/Field/Walls vs others).
If you have this info, it will be much easier for you to defend the base even when you are outnumbered 3 to 1.
 How to Defend
If you will need to defend, and thus the enemy will have the filed you will need to know how to defend. One can defend easy even if your new! But you need to know How/What/Where and Why.
First off mines, Proximity mines to be precise. These will guard your builings and stall the enemy even if your gone. perhaps not for long, but every second counts! Mines can be disarmed but don't underestimate them. They will in most cases kill of an enemy if placed properly. Defending is mostly needed when base defenses are destroyed or disabled, then mines should be placed in common openings to base to prevent stealth nukes if you're in GDI. These can be tunnels, or in Hourglass from either side of your (destroyed) tower to the mini-bunkers. If you're mining is below the limit still, mine up the wall hopping place if you can.
Mines are also good for protecting against tanks (if you can place enough in 1 place!). But this will be discussed later!
For more information about mining clearly read the Advanced Mining tutorial.
Infantry can defend a base perfectly. But this means a mix of units and well placed ones. Place your snipers in spots where they are hard to see, but still cover a wide range. make sure they can see each other so they can barrage other units or aircrafts.
Anti-tank infantry is also needed, but these can patrol the ground in order to spot enemy tanks. Always keep those men in groups of 2 so that if one is weakened the other can provide cover, or take down the tank.
It is essential for infantry to avoid open spaces! Enemy tanks tend to drive you over rather easy... And Apc's especially can be a pain in the behind. A few fast firing units should also stay with the ground, in order to constantly do damage to the enemy. This way the engeneers of the enemy will have a hard time repairing the damage.
Speaking of repair, a group of 2 or more engeneers should stay in base in case the worst should happen "Building under Attack!". If that does happen, the engees can always be there to repair it or defuse nukes! Often people don't listen to commands. This can mean very worse things for your team, like the Refinery getting destroyed or another important building. Make sure that you don't get too far away from the base, and make sure that you always have a scout that can spot incomming tanks or infantry rushes.
Able to take more damage then infantry, tanks can be very useful for defending your base. Especially for Nod with their slow Obilisk, a well driven APC can block the path of 3 medium tanks (like on Field) long enough for the Obilisk to take out all three, rather than just 1 and a half. Try to keep your tank near an obstacle or wall that you can retreat behind to avoid damage, or where a friendly engineer/tech/hotwire can hide to keep you well-repaired.
Anti-infantry tanks: These are excellent for killing infantry rushes on maps with no base defenses. Do not leave your base without one. However, they are generally not needed on maps with base defenses (some maps where GDI infantry can avoid the OB could be exceptions, but mines are better suited to dealing with this.
Anti-tank tanks: If your enemy is coming in with tanks, you better have some tanks to respond with. Artillery/MRLS, and stealth tanks are not good choices for defense due to low defense, and flame tanks' low range makes them less appropriate, but light tanks, med tanks, and mammoth tanks are excellent defenders. The idea here is to cause as much damage as possible to your attackers while staying alive and allowing any anti-tank infantry to do their work unharmed.
If you are having problems with ioners/nukers a lot, an apc on patrol can be useful, especially to weed out those SBHs, but unless you are a defender with nothing better to do, your time is generally better spent doing something more damaging to your enemy.
 Base defenses
These will already defend your base, but they need to be defended, because they are not invincible. Each defence structure can be taken out! That is why we have created a few topics especially about the defences!
 If they got a building
If the enemy got a building, one must reconsider there defense! Either make sure you defeat the enemy before they reach your base, or create a good defense.
Sometimes, one needs to mine outside the buildings in order to get victory. Not to kill tanks, but to search for stanks, and SBH! A small line of mines between 2 buildings, can sometimes protect the building which is behind it.
This picture was taken in Gobi - a small map only for infantry. What you see here is an example of the technique. You may say "Damn n00b place the mines in the door!". If the Barracks and the Refinery are dead, and the Power Plant is alive! Then what? Place mines around the entire place to try and catch an sbh?
No, by placing the mines like this, you create a "Safe Zone" these zones if a nuke is placed, or when you are attacked will be 100% safe as you will easy notice if someone has entered it. Sbh will also not try and nuke in a "Safe Zone" as this will severely damage them, so this means there nuke ground has shrunk to around 50% but the changes of the enemy catching them raises with 50%.
Another example is on Mesa. GDI will mine the small path near there PP in order that if Nod does a flame rush, the first tank will be either dead or heavily damaged.
For info about how to best defend a building: Structural guides of buildings in Renegade
 How strong must my defense force be?
First of all, 30 people attacking and 5 defending doesn't seem fair, if something gets destroyed at base, the attackers could blame the 5 people. A balance must be created, since attacking is a high priority, it should be given more people, but not too many; if it's a 35 people team at least 25 should attack while the other 10 defend.
Depending on the amount of people defending they should be distributed in strategic positions, like tunnels; if it's a flying map someone should be constantly revisiting the top of the buildings to avoid roof beaconing. There must be a scout in a position where he can see the vehicles or infantry that are approaching, the scout should preferably be a sniper (due to his long range scope and the capability of slowing down infantry groups). The scout should always tell the team what is aproaching and from where.
Mining is very important, check the Advanced Mining guide for more info. Among the people defending the base there should be at least 3 or 4 snipers, 2 or 3 heavy infantry (Raveshaw, Prototype Assault Suit) and at least 5 light infantry (Patch, Nod Chaingunner). If everybody is light infantry they won't be able to stop a vehicle attack. If they're all heavy infantry, they won't be able to repel the light infantry that attack them (mainly because heavy infantry weapons take longer to reload and can be easily avoided by light infantry classes). And the key element of defending a base are engineers, at least 1 or 2 should be repairing each of the buildings.
The distribution of the defenders is also very important. Heavy infantry classes should be put where vehicles can enter. Snipers should be positioned in places where they can look easily at most of the buildings. Light infantry classes should be disperse around the base, near buildings or protecting the infantry entrances (tunnels). If the team is okay with leaving a vehicle to defend the base (not one too slow such as the Mammoth Tank or Flame Tank), instead one that can easily manuever and be able to kill or block vehicles and infantry [eg: Medium Tank, Light Tank, APC] it should be stationed near the middle of the base, where the driver can easily manuever to each of the buildings and where it's able to shoot enemy vehicles entering the base.
If all of this is taken in consideration or followed during a match, the base should be perfectly safe and the match will be easier to win.