Save them all! Sapphires will prove very useful, multiplying your offense and defense.
In the Black Market, you can buy something called Legendary units. Legendary units will boost the offensive or defensive troops that you own up to 30 times, but Legendaries do not work alone.
Read the description of each, and see the units they support. The first Legendary you may want to buy is the Hero. As soon as you have 50 Infantry offense units, go get yourself a Hero.
Sending the Hero along with 50 Pikes or Pallies will let them strike as hard as sending 100 of them with no Hero. The same will happen with all the other Legendaries; if you have the troops to go along with them then buy them and use them.
During the first 30 levels, you can save up some 2500 Sapphires or so. Use them wisely.
Another thing you don't want to miss is the Altar of Weor, you can build this one in Construction -- Resources for 1000 Sapphires. Then you upgrade it 6 times for free (each upgrade takes 24 hours) and at level 6 it will boost your gold and Iron production by 25%.
The tasks that pop at the left of your screen will guide you throughout the game, and you will be getting good rewards out of them. The higher the level, the more resources you need, and the better the reward.
In order to not miss any of the rewards, try not to do things before you are asked to do them, because if you do things ahead of time you won't collect the rewards.
If you need more food, you can upgrade 3 farms as high as necessary to meet your resource needs, so that when the game asks you to upgrade 4 farms to lvl X you will need to upgrade only one to collect the prize. The same thing happens with Mines and Towns, just upgrade a maximum of 3 until the game asks you to go for the 4th. Then once you have upgraded the 4th, you can upgrade the remaining ones.
Always aim to produce at least twice as much food as you are producing gold and iron, i.e. if you are producing 200 gold per hour, you should be producing 400 food per hour. Gold and iron production should be balanced.
Alliances are not useful if your ally is 24 hours away. You need to ally with your surrounding neighbors. You will receive news of their scoutings and raidings, take advantage of them for your own raidings.
Try to spot strong players, talk to them and ask for an alliance, especially if they belong to other League: you don't need to make alliances within the League members, they are already allies.
Then you can team up with your allies to take down stronger enemies and enjoy fast resource and scroll trading.
Easy! Go raid one (preferably lower levels than yours, your own level if you followed tip number 1 should be safe) and look around for signs of strength (Walls, Castle guards, High raid points or Offense points, etc). If you spot a strong one, don't mess with it...you will most likely lose, better to try to make friends with them.
As soon as your raid is successful and you have killed all the defending units, place a Siege with only 1 archer.
If the owner doesn't liberate their castle, that castle is abandoned and yours to claim in the name of your league. You can either farm from it or you can raid it once a day. If someone else raids it, just re-Siege it.
Always try to get your current Lost Arts to lvl 20 before discovering a new one. If you discover several first and try to upgrade later, you will end up with a weak army. Better that all the troops you can build are as strong as possible..
Build your army as much as you can, every day. The more troops you have the stronger you will get and the easier it will be for you to dominate your area and assist your League mates and participate in League military events. If your food production gets too low, use your army in Battlegrounds, Playing Battlegrounds is always an investment that eventually will give you profit.
Try to arrange your army by raiding parties, so that you can launch 10 raids at any given time. Raiding is where you will get most of your resources from and it will allow you to build more troops and run upgrades very fast. Work on your storage so that you can start aiming for 50K raids.
While you are building up your army to attack others, do not invest resources in defending your castle. Simply send your units to the Catacombs and spend all your resources when you suspect trouble is on the way.
Regardless of how strong your potential enemies might be, they can't harm you if your troops are secured in the Catacombs and they can't take resources from you if you spend them all before you log off.