Some pointers on Clash of Clans strategy

Offence in War

If you suspect there is a trap where you want to deploy troops? Or maybe there are bombs? Spend goblins first. They are too fast for traps and bombs and are capable of disarming a mine field.

Healers are effective. Consider having two per full army. Healers are susceptible to seeking air mines. Consider releasing a bunch of minions first, to clear the airfield, and then send healers after them.

Use better troops (not necessarily more expensive troops, but a lot of times more expensive troops means better troops). Instread of archers, use wizards, for example.

The first thing you need to do is lure the opponents' defence troops out of the castle and destroy them. This should be done before the majority of troops are moved in. Therefore, some portion of your army should be troops that are capable of destroying other troops (giants and hog riders, for example, and not capable of doing so). I prefer to use minions for this purpose.

Use rage spells instead of lightning. They result in higher average destruction. If you have doubts - take the number of defence buildings your first batch of giants destroyed, and divide that by three. That's how many buildings a rage spell can help destroy (instead of just one or almost one, by a lightning spell).

Scoring stars

It is very important to understand how the scoring system works. Many more times than once have I seen people attack the wrong kind of opponent and lose the war - all out of best intentions.

You want to maximize the total number of stars scored by your clan. That doesn't mean maximizing the number of stars you score! If there is opponent who nobody but you can one-star, and all other opponents are 3-starrable by someone else, you should bring that one star to the team.

Take the highest opponent that you think you can 3-star (call him X). Take the highest opponent that you think you can 2-star (call him Y). If nobody can 3-star Y, you should hit Y. Is someone else can 3-star X, you should also hit Y. Therefore, you are maximizing the number of stars that you clan is receiving in war.

Also. There are top clan members and lowsies. Lowsies are generally encouraged to attack first, so that the top level members can figure out the best course of action considering the stronger opponents. It's bad for the clan if attacks by higher castles are used to three-star inferior opponents that should have been taken care of by some lower-level castle.

Defence

Walls

When positioning walls, I like to think about connectedness and enclosure size minimization (partition-ness).

Walls are important. Let's say you are surrounding a building with walls. The enclosure is only as strong as the weakest wallpiece. Therefore, walls should be upgraded uniformly, lowest level walls first.

How many walls does your opponent have to break to get to your townhall? That number will be usually between 0-4. When positioning walls, consider the number of wall lines that separate your important structures from the edge of the field.

Next, let's say you have four defence structures: an archer tower, wizard tower, mortar, and air missiles. They are in one enclosure. Well, it's more difficult to destroy these structures if there is an inner wall, i.e. if they are in two smaller enclosures. This is what I'd call partition-ness: it's better to have many small enclosures than few large ones.

Then of course, you run out of walls (you may have no more than 270 wall pieces total). So you have to come up with a layout that maximizes the utility of available walls.

If you are building your base radially-symmetrically, consider having smaller enclosures in the middle of your base, than on the periphery. Put 1-2 buildings per enclosure in the middle, and 3-5 buildings on the periphery. You need a shorter line to partition enclosures in the center, therefore it is more efficient to have smaller partitions in the center.

Lastly, consider when braking a single particular wall piece, how many enclosures your opponent gets access to? If you look over your base you may spot such a wall piece that, if broken, makes 4 enclosures into one. And the majority of walls, if brooken, should only allow opponent access to 1-2 enclosures. Therefore, you can reinforce the corner wall pieces or add doubles such as to reduce the number of these `weak links` that connect many enclosures. In the following screenshot, notice that internal corners have been reinforced for that reason.

What constitutes an important building?

You can have war castle different from the peacetime castle. Generally, in war you are interested in scoring as many stats as possible, while minimizing the stars your opponents can score on your base. Therefore the most interesting building in war is your town hall. In peacetime, I see mostly people being concerned with preservation of resources. Since the townhall holds few resources, the most important building in peacetime becomes the dark elixir storage (and other storages). The war base and the peace base differ fundamentally in that the war base protects the castle, whereas the peace base protects the resources.

Air defence

Air defence is effective against healers and dragons. A lot of times it happens that your opponent spends special effort to take out your air defence, and sends in a healer. Consider putting seeking air mines right next to the air missiles. But to psychology of your opponent, this move has high probability of taking out the attacker's healer right away.

Consider also that air mines and air bombs should not all be on the periphery. You don't know from which side you may get attacked by dragons, and against that, having centrally-positioned air mines / bombs is effective.

Various

Mortars are valuable and should be carefully protected.

I like to stick one or two archer towers right in the middle of the base. Their long range means that while the opponent is being busy destroying whatever structures, a centrally-positioned archer tower can be reducing enemy troops. Whereas if you put a wizard tower there, its short range means that while the opponent destroys nearby structures, the wizard tower may actually stay idle.

Giant bombs to 4x damage on pig riders. Position a few giant bombs next to the defence structures that are likely to be hit by pig riders first.

I like to put traps in the middle of the walls where enemy troops (particularly giants) are likely to go through.

Do not protect gold/elixir storage with traps. Traps are most effeective on giants, so their best applicability is protecting defence structures from giants.

Put buildings in all four corners of the field. If your opponent is very strong and is sure to defeat you but he doesn't have enough time to rush his army all the way to the other side of the field to destroy that last one builder's hut - with 98% destruction instread of 100%, you saved yourself a star, which in war makes a lot of difference.