Survival mode is all about defending for as long as possible, but eventually you will be overrun! You can play with up to 3 friends. Some players have scored up to an hour and 30 minutes - but an hour is perhaps a more realistic target to set yourself. The main difficulty spikes in survival tend to occur at 20-25 minutes, 40 minutes and 60 minutes. The spike at 20 minutes happens when the first "leader" creeps start to appear, and your setup will face its first major test. Assuming you survive that in good shape, you can continue adding to your defence, and will hopefully be reasonably comfortable for another 20 minutes or so. At this point, the creeps are getting a lot tougher and faster, and any deficencies in your defence will be shown up. Generally as long as you have a good mix of turrets and plenty of utility turrets this can be rode out. At about an hour the difficulty takes a massive uphill spike, and it becomes extremely difficult to survive any longer due to the sheer speed and toughness of the leader creeps.
A quick note on support powers - as the game progresses even the levelled up damage support strikes (e.g. Firestorm, Nuke etc) become increasingly ineffective. If possible take both rage and overdrive as your turrets can inflict massively more damage at this point.
Surveying the Map
Take a good look at the map when you play, and try to decide on the best place to position your kill zone(s). Different maps lend themselves to different tactics and turrets, but it's generally best to concentrate as much of your fire as possible in a few key areas. Generally these are areas where roads crossover or converge so your turrets hit as many enemies as possible. Some creeps regenerate their health or shields while not being damaged, so you want to kill them entirely as soon as possible - although you can get around this issue through careful placement of inhibitor towers.
All turrets (especially radiation and other area of effect turrets) work best in combination with slow effects so they can hit the creep as many times as possible while they're in range. All pre-made commanders offer a selection of turrets with different ranges (although this varies) and you can of course go custom. If you go custom, you want a selection of turret damage types, and a variety of ranges depending on the map. Personally, I choose one of each damage type, plus two slowing turrets and a boost for most maps. Choosing a selection of ranges allows you to position your turrets so that each is at their optimum radius and aren't competing with each other for space.
My personal selection for most maps is:
Tesla | Radiation | Pillbox | Missiles | Slow Goo | Turbulence | Speed Boost
But everyone has their own favourites, and there are many different ways to play the game - that's part of the fun! As you add more players you get a better selection of turrets, although the difficulty is increased to compensate for this.
There are several ways to breakdown how the turrets work, but the first is what kind of damage they inflict:
Do equal damage to all creep types, have a slowing effect.
Do extra damage to mechanical, but less to cybernetic.
Do extra damage to biological, but less to mechanical.
Do extra damage to cybernetic, but less to biological.
Do equal damage to all types, but are weak, and have an extra effect.
For this reason, you usually want a good mix of all damage types and to try and balance the number/quality of each type. Different turrets also do either single target or area of effect damage. Generally area of effect attacks are weaker, but hit multiple targets, while single shot do more damage but only against one target. You need a good mix of both, but in order to get further into survival mode, single target turrets become more important against the leader creeps. Some turrets also have an additional slowing effect, and these are very important as time passes. Balanced against this, these turrets tend to do less damage overall.
Boost turrets are also vital in survival mode, as they improve the damage causing capability of surrounding turrets. These effects "stack" so that a turret next to more than one boost will get the full effect of all boosts present. Be sure most of these turrets are upgraded to at least lvl 2 in order to make the boost effective. For a quick example of this, we'll use some laser turrets and a speed boost:
Lvl 1 laser turret does 148 damage per second Lvl 2 laser turret does 643 damage per second Speed boost increases damage by 30%
8 lvl 1 laser turrets do 148 x 8 = 1184 dps With a speed boost that costs 150, they now do 1184 * 1.3 = 1539 dps
7 lvl 1 laser turrets do 148 x 7 = 1036 dps 1 lvl 2 laser turret does 643 x 1 = 643 dps Total = 1679 dps
And the upgrade to lvl 2 actually costs less than the speed boost. Later on in Survival mode, good boost use is crucial to success - they are significantly less expensive than Level 3 turrets, and if placed correctly do more "damage" (through the turret boosts) for the cost. The simplest example of this is that a speed boost effecting 4 turrets will do 120% (4 x 30%) damage, compared to 100% for a standard level 3 - and at a cost of 150 each instead of ~350.
There are many ways of stacking your boosts, here are a few examples:
(T = Turret, B = Boost)
Useful early on, but they show limitations later as they are less cost effective than other methods. The Grid:
Better than the square, but still not as good as the next examples.
More efficient than the standard square, each boost will effect 4-6 turrets. The turret in the middle receives 8(!) boosts!
A smaller variation of the mega-square, not quite as efficient, but still effective.
Lattices are extremely effective, and easier in some ways to expand than mega-squares. The maths involved to work out which is more efficient is pretty mind boggling, and depends a great deal on the kind of turrets and terrain involved.
Does a small amount of damage to ground units, but its main effect is to slow them down. Good goo placement is vital for ensuring your other turrets get to inflict as much damage as possible. A good tactic on maps with multiple converging roads is to overstack one of them with goo (or other slow turrets) so that the waves are more split up when they arrive in your killzone.
A strong slow turret that only affects air units. Bring these or flak to take down fliers; taking both is usually overkill.
Although it claims to be good against shielded opposition, it is actually pretty ineffective at taking down shields unless massed. It is good at slowing down mechanical and cybernetic ground units though, and should be used in a similar way to goo.
Inhibitor towers have a variety of effects - they reveal cloaked units and also stop creeps from regenerating shields and health. Use them if you want to avoid the micromanagement and nuisance associated with flares.
Speed boosts are the best general purpose boost, as all non-utility turrents benefit greatly from their effect (+30% fire rate). Speed boost is recommended for "constant fire" weapons such as teslas, machine guns, anti-air, radiation, lasers, pillboxes and drones. However, it does work effectively on all the other damage turrets. Stack multiple speed boosts in a grid pattern for best effect.
Critical boosts are very effective on the slower firing turrets, but not great on the "constant fire" weapons mentioned above as their individual shots do low damage. They give a 25% chance per shot do a critical (x2 damage), and the damage added to "constant fire" weapons is very small compared to the slower firing turrets. Criticals work best with shotguns, lightning bolts, silos and railguns. They help mortars, missiles and cannons to a lesser degree.
Range boosts are very useful in certain situations, but work best with area of effect weapons where they actually boost damage potential rather than just letting them fire further. The reason for this is that later on in a survival game, pretty much all your turrets are firing constantly and the additional range is less effective than one of the other boosts. Aside from specific turrets on specific maps, range boosts generally help teslas, radiation, shotguns and machine guns the most. This is because these area of effect weapons will now hurt more creeps for longer with their extended range.
On some maps, particularly later on, you may have to build in less optimium areas and here range boosts can be even more useful, and ensure all of your turrets are firing as often as possible.
Each power station allows you to store +5 power, and boosts your energy income. These are useful, but difficult to fit into your loadout in a single player game. They require a lot of up-front investment; if you take them, build them later on once your front line is stable or you may find yourself overwhelmed by the early waves.
Shotguns attack both ground and air creeps, and pack a heavy short range punch. They do damage to everything within their blast, making them a useful area of effect weapon. This is balanced by their short range, which tends to make them useful only on particular maps. Shotguns work best on the outside of a corner, where they can fire directly into a crowd of creeps as they approach and exit the corner. Boost priority: Critical, Range, Speed.
Cannons are cheap and well balanced turrets, attacking both ground and air creeps. They fire in bursts, letting them benefit from both speed and critical boosts. Cannons work best at medium range on the inside of a corner section. Boost priority: Critical or Speed.
Machineguns are quite similar to shotguns, with slightly longer range but no anti-air capability. They fire a constant stream of fire rather than bursts, and have an area of effect at everything within their cone of fire. They work best in similar positions to shotguns, on the outside of a curve. Boost priority: Speed or Range.
Pillboxes are actually one of the most damaging turrets in the game, once fully upgraded (~3000 damage per second at lvl 3). Their scattered targeting means they struggle with boss creeps unless focus fire is used constantly, so don't rely too heavily on them. They have a fair range, and work best on the inside of corner sections in a similar way to cannons. They have no anti-air capabilities. Boost priority: Speed.
Railguns are slow firing, slow turning and have fair damage capabilities. They fire at both air and ground units and have good range. The big benefit of railguns in that they ignore enemy shields, bypassing them entirely. However, this is difficult to use in combination with other turrets which will damage the shields first and makes them hard to use effectively. Railguns are virtually useless if built in small quantities; if you bring them, they should be the cornerstone of your strategy, with a good selection of support turrets (such as critical boosters, slow goo and radiation) to back them up. When used in large numbers, they are extremely deadly and will make short work of tough shielded enemies like tanks and Zorasts. Boost priority: Critical, Speed.
Anti-air, as the name implies, does not attack ground units. It is the single most damaging turrets in the game (~3300 damage per second at lvl 3), and also does a small amount of splash damage. Place them in the front of your line so that your other turrets don't have to waste shots on fliers. In single player, flak or turbulence will generally be enough to stop air units so taking both is usually not necessary. Boost priority: Speed.
Teslas are very useful turrets, causing good damage and slow down against ground units within their short range. Teslas work well on levels with lots of thin roads and sharp corners where you can get the maximum use of their short range - they're much less effective on maps with wide roads. Boost priority: Range or Speed.
Very useful general purpose turrets, they have a long range and hit both ground and air units. Although they do good damage, the major benefit of lightning bolts is they do at least 50% slowdown on any units they hit, and they will deliberately try to shoot units which aren't currently slowed by them. This makes them extremely useful against the leader creeps you'll encounter as the game progresses. Boost priority: Speed, Critical.
Flamers are a unique turret which only effects ground units, and only unshielded ones at that. Flamers set on fire any unshielded ground units, and the major damage they cause is through this ongoing burning rather than their actual attack. Flamers are best used at the "front" of the map nearest the dropships, and you should spread them out so that the enemy is constantly on fire until it reaches your main killzone. Flamethrowers use a special damage model - damage-over-time from different turret levels stack, but not from turrets of the same level. While you might be tempted to use this property by building a L1, L2, and L3 turret in close proximity to get damage from all three, the L3 turret completely dwarfs the damage from the other two turrets. Boosts are generally ineffective on them.
Radiation turrets are very useful, as they damage all units within their area of effect, and also make creeps more vulnerable to damage from other turrets. Radiation works best in your main killzone where it will assist your other turrets, and on the inside of corners and loops where it will effect a large area. Boost priority: Speed or Range.
Lasers are well balanced, inflicting heavy damage on both air and ground units. They work best on the inside of curves where they have units in range for as long as possible, although they will only shoot one target at a time, so they should be placed behind your area of effect turrets to avoid wasting damage. Boost priority: Speed.
Missiles are probably the best all round turret in the game, they combine good damage, range, anti-air, anti-ground, slow and area of effect. The slow effect and splash damage are quite small, but worthwhile. Fairly fast firing so gain slightly more from speed than critical boosts. Boost priority: Speed, Critical.
Mortars only effect ground units, but have good range, slow and area of effect capabilities. Mortars fire quickly and are excellent at softening up incoming waves, but struggle to kill specific targets due to their relatively low base damage. Boost priority: Critical, Speed.
Drones are an interesting unit with good damage and the ability to follow units far out of their range. This makes it a little tricky to get the best out of drones as they don't keep their fire within your killzone, but they can weaken units before they arrive and chase down any stragglers who make it through. Drones only need a unit to pass through their range in order to be triggered and follow the unit, so can be placed well back from roads with their range only brushing the middle. Boost priority: Speed.
The silos all have a very similar purpose, differing only in the kind of damage they inflict. Silos are difficult to get to grips with, their long range means you want to place them very carefully - ideally so they are only triggered to fire as creeps enter your killzone. They are excellent for crowd control and killing stragglers, but only when used en masse. Boost priority: Critical, Speed.