Walling In
Walling in your town is useful in the early game, but is easily defeated later on. It prevents early grunt attacks and lets the game extend to the middle part of the game. Always have a tower set up behind your farms The problem with wallins is that when you need to expand, you can't easily get your peasants out. Also, the enemy can just set up many towers outside your town so you can't get out.
Wallins are easily taken down later on with a few well-placed sappers or a blizzard.
A good idea for wallins is to leave a hole and when you see an attack coming plug it up. This way you can get your guys out if no attack comes, but still be safe from attacks if need be.
Here's a good example of what a wallin should look like:
This way your towers protect your barracks and smith. You can also get out your troops to attack them. The key of a wallin is to keep your enemy from getting through it, so you need to try to keep them from getting sappers or mages.
This next way is not really a wall-in of your town, but of part of the map. This was done in a game on POS.
When done early on, this is almost impossible to break through. Notice how the towers are a step back. This way, if someone brings ballistas up, your ballistas can move in and kill them quickly. The peasant is also kept to repair.
This was done in the middle of the map on the right side. I can still bring guys around the middle to attack their town, which is right above this.
Another form of wallin is the NaTe formation. It doesn't wall off your entire town, but just a tower.
This will keep your peasants well protected from attacks. Once again, it is useless after sappers and mages. A mage can rip this and part of your TH down in one blizzard.
This can be upgraded as a cannon or guard. A cannon will be best if you have the money. You can replace where the peasants are with walls, to prevent sappers from taking out more than one farm.