[Potential Problems]
But this is good for me, because Apprentice's main problem is a simple one: surviving the first wave. You better hope you picked up at least a serviceable weapon, or a serviceable companion, because the mana they give you to start out (depending on the level) might not be enough to get by.
The other problem you might face is at the final waves. Ogres, ninjas, multiple ogres - whatever is thrown at you might be too much at this stage. Ninjas or ranged folks could distract you from repairing towers and blockades, and multiple ogres could hit two sides of your defenses at once. You can mitigate by equipping the engineer, enlisting an all-DPS friend, or a repair-capable friend.
Your final thing to watch out for: The towers have vertical range! Try to watch this while laying towers down, they will not target enemies too far in the up and down direction! Also, if there IS a ledge or something in the way, they will sometimes not see enemies, or hit the ledge while trying to hit them. Even if the ground is yellow when you're putting towers down, try to be reasonable about things being in the way of the tower's shot.
[Gameplay]
Other than that, the middle waves and in teams is where you're going to shine. I almost always build as close to the crystal as I can (without allowing the possibility of archers standing outside my range and hitting the crystal) because I want the lightning circles to cover the crystal in case wyverns or ninjas sneak through. Place fireball / deadly strikers behind the crystals, firing past them, and don't forget to cover the same area with fire as well as lightning, for those that are resistant. You don't want a lightning-resistant archer hitting your towers from outside of your fireball towers' range! Once you're entrenched, all your friends are free to run around and take care of enemies as they see fit, without having to worry about the crystals. On boss levels, by the time the boss arrives, I have every lane on lockdown and everyone on my team can focus the boss without worrying about any of the hundreds of minions reaching the crystal(s).
Usually my setup is focused around the lightning tower - though the range is listed as the smallest, it will ultimately hit enemies the farthest away due it it's chain. It will hit enemies around corners, it will hit enemies from miles off. As long as there are hordes (and there are always hordes) it will be your main source of killing before mobs even reach your barricades. It even stuns them for a good moment. The only thing you want to watch for is it's health, because it has to be a little closer to the barricades than other towers, it can get splash damage from exploding kobolds and will definitely take damage from ogre attacks. It could go down during those waves if your back is turned.
Lightning towers of course need to be protected by blockades. Your blockades, if you're not playing with a squire friend. A squire TOWER friend, too, because a DPS squire's blockades and slice n' dices will actually not be as tough as yours, if you've put anything into tower health (or have the engineer equipped). You can't spend all your time trying to repair his blockades, even if they are doing damage, you're better off with the magic pen that yours give and the increased health.
Lightning towers need to be backed up by fireball towers, because lightning resistant mobs will come through. You will have to place fireball towers where they cover the area in front of the blockade, and if possible, the areas where lightning resistant archers could stand and hit the lightning tower.
Finally, the deadly strikers, if you have the Defense Units to spare (your friends may be taking them up, and they are 9 each). It's level-dependent, but I often have them behind the crystals, firing across, towards where the mobs or wyverns are coming from, to pick off anyone who could get to the crystal, and hopefully do some damage to ogres when they arrrive.
[Stats]
So far I have 50+ in all tower categories, and the rest of the points are going to go into attack and attack speed. (I have absolutely zero points in player health, I can be killed in probably two shots, even with bonus health from items) Fifty points in range really expands the tower's range to a very respectable distance - almost 120 degrees for fireball towers, and a solid 45-60 degrees on the deadly striker, and a distance range that can't be seen by the tower camera (they need to make that camera bigger).
