These are not the best tips, and if you really want to acquaint yourself better to the game mechanics, you should definitely check out the official KAG forums and just read up on pro-tips there. But anyway, for beginners this is a list of tips that I've found help me out, and will probably come in handy for our upcoming KAG tournament...
When the game begins there should be a core team who runs to the frontlines and covers as much ground as possible. Especially on flat land maps, or maps with a giant hill in the middle, this is a very VERY important strategy. The more land you can occupy, the better chances you'll have of winning. If the team is big enough, half the builders should stay behind and strengthen home base defenses, while the other team should lead the charge and build on the frontlines with the help of knights and archers. At least 2 builders, 2 knights, and 1 archer would be an ideal frontline attack providing that your team is big enough to support that.
On severely flat maps, rush attacks should be expected. Don't sit around building traps and fancy towers, your enemy is going to just blitz the crap out of you. Instead, try to blitz them first. A builder should accompany a team of knights and use ladders to build fast pathways over enemy structures. Once you've gotten as far as you can go, the builder should either hijack an enemy tower, or build their own really quickly. Then they can go back and start dismantling, or repurposing all the stuff they covered in the blitz. Again: The more land you can occupy, the better chances you'll have of winning.
Build towers at least 7 blocks high and 10 blocks apart if possible. Knights are tricky buggers and they can jump much higher than other classes. Block them off by building tall towers and not using wood, as a Knight can slash through wood and break it. If you have too many towers too close together, when a knight does get on top of your tower he can easily skip across all your carefully laid traps. So keep towers separated with enough space to prevent them from doing that.
Do not build 1 block wide towers. Just... DON'T. They are notoriously fragile and one really strong hit from a catapult, or a quick smash from an enemy builder, can topple it leaving yourself open and possibly crushing both you and your teammates. Build towers at least 2-3 blocks wide and reinforce with castle walls.
Try not to build towers with door footholds. I know it makes for easy climbing for your team, but opposing knights can slash at the doors, break them, then use the empty space as footholding to scramble up the wall quickly. Best to use trapdoors instead.
Don't build pits. Pits take way too long to build, and tend to be rather ineffective. The only reason to build a pit is because there's a lot of stone and by harvesting it you build one naturally.
Your time is better spent building castle walls at the front of the map. Don't stay back and "prettify" your home base. You'll be wasting valuable resources and not aiding your team. That doesn't mean you shouldn't strengthen your keep, but just don't spend too long back there. Keep the battle at the front of the map if possible.
Don't stand in the way of builders. Chances are they're trying to make repairs or strengthen defenses.
Find gold immediately. Gold is essential in providing you with a workshop to build catapults and outposts. With a catapult you can attack your enemy from a long distance, and fling knights into enemy territory to do damage. With an outpost you can bring your troops closer to the frontlines in case they die, resulting in less ground they need to recover before reaching the heart of the battle. It just makes things more efficient for your team.
Builders should set up a series of shops for the knights and archers in each key spot they can. It's very frustrating, and inefficient, for troops to have to keep falling back to get resupplied. Keep the shops moving close and closer to the frontlines.
Tunneling is not very effective, so it's recommended you don't do it. It takes a long time and one lone knight can ruin everything, plus it can result in the enemy taking over the tunnel and getting a bee-line right into your base. If you're going to tunnel, make it short under the enemy towers to either collapse them, or get through their defenses.
Knights should try to protect builders whenever they can. Builders have to stand motionless to build, making them an ideal target for archers and enemy knights. But a good builder can mean the difference in a rough match. Stick with a builder and keep him safe so he can strengthen defenses and build shops to help the team.
Knights should also learn the 3 basic things they are good at. Attacking with a charge attack (which can take out doors and do significant damage), throwing and deflecting bombs, and using their shields as a parachute to slow descents from high jumps.
Archers... just... keep with the flow and rain a shower of arrows on the enemy as often as possible. Stay hidden in the tree tops and make sure to vacate as soon as enemies come closer as they can knock you out of trees with a well placed mallet strike.
NEVER put ladders on the side facing the enemy.
Most of this should be just common sense stuff, but I'm always baffled by how many players don't seem to "get" these really basic things. And, like I said, these are not the best tips, just ones that I (playing as a builder) have found to be the most helpful.
Our previous videos:
Introduction and Tutorial: [link]
Kag Match 1 with Dev team member: [link]