Building sequence can determine the outcome of the game. This plan works quite well on Easy and Normal difficulties, otherways it may take longer to complete.
(Day 1) Town Hall (gives 500 extra gold per day)
Capture wood and ore mines as soon as you can, wood is the first priority for this building strategy.
(Day 2) Mage Guild (spellbooks to heroes; required for City Hall)
(Day 3) Blacksmith (First Aid Tent is useful in the beginning, but don't buy Ammo Carts; required for City Hall)
(Day 4) Marketplace (has no use in the beginning; required for City Hall)
(Day 5) City Hall (gives 1000 extra gold per day)
(Day 6) Citadel (extra town defense; +50% population; required for Capitol)
(Day 7) You'll have to decide here: either build a Castle (extra town defense; +50% population; required for Capitol)
or build another Creature Dwelling (to have something to recruit next week)
(Week 2) Build Castle if you haven't, then save money and build Capitol as soon as you can.
Now you have an income of 4000 gold per day, 200% population growth and castle walls with 3 arrow towers!
Once you have the Capitol, have your town well defended, you don't want to face your walls nor spend 10000g on Capitol again!
(Capitol is destroyed if the town has been captured by an enemy who already has a capitol).
Next thing to do is to build all Creature Dwellings as soon as possible. Build the most important ones first and don't upgrade until you've got all 7 creature dwellings! The reason is to have the highest number of creatures possible.
Another important building on Week 3 is the resource silo, the sooner you build it, the more resources you'll get. If your town is rampart, think about Mystic Pond and Treasury around Week 3. Before upgrading Creature Dwellings, build all the special structures which increase population of particular dwellings (ex. Cages or Griffin Bastion or Mess Hall).
Okay, now you're ready to kick some butt! Build/upgrade the rest of structures as you please.
HINTS AND TIPS
When your castle is about to be attacked and you have only about 1000 gold, spend it all on lvl1 units, then divide those units into 7 stacks. You will still lose, but arrow towers will have more turns and will hurt your enemy a bit more. Same applies to heroes, you will have more time to harm your enemy with offensive spells before running away and your one or two stacks will not be killed before you can run away.
If one of your cities or towns is going to be captured, buy up all your remaining armies and either leave them (in this case recruit a hero to get those little spell and attack/defense advantages) or run away with them to give to a stronger hero. NEVER leave armies for the enemy to recruit. Often it is better to take a city over a hero. A hero needs armies in order to survive a city can always hire a new hero.
If an enemy hero is nearby jump into the nearest castle. This will defend your armies better giving you the ultimate advantage.
Always capture mines even if you have heaps of resources: few Marketplaces will give you about 150 gold for one unit of resource - every mine is a gold mine! Also, you will prevent enemy from building some expensive troops and structures. Try to build as many market places is possible before trading. If you have less than 3 marketplaces in towns, it's a good idea to visit marketplace map location.
When you pass by cover of darkness, be sure to visit it - large area around will be shrouded on enemy's adventure map. Believe me, cover of darkness on the map or in Necropolis town is one of the most annoying things that can happen.
Necropolis towns have a great strength / weakness when using the "cover of darkness". It is mentioned on other tips that the cover of darkness is very beneficial to the user to "hide from others." However, it unfortunately gives away the location of the Castle. By looking at the circle it creates, even if you can only see a small portion of it, is all you need to b-line it to your enemy's location. Also, if a non-castle cover of darkness is used, you at least know of the position of the enemy, a very powerful bit of knowledge.
Gold mines should be taken as soon as possible. If you need sulfur for dragons or crystal or whatever for the highest level creature, do everything in your power to get IT. Sometimes it is well worth trading a lot of wood for a few resources.
If you are playing with an ally, DO NOT BE GREEDY if you have a resource you don't need give it to your partner this strengthens both of you especially if you have different cities. You may not even need any sulfur at all where as she/he may need a lot.
Know what the artifacts are by sight, some artifacts are worth losing many armies over and others are just useless nice-looking pieces of jewelry. Age of Heroes has complete illustrated list of artifacts for you to learn.
EVERY week check your tavern to see the new heroes to buy. There may be higher ranking heroes that have died in combat. They may even have some artifacts in their backpack. Also look for heroes that have estates or 350 gold per day or any ANY resources buy them as soon as possible, its like a walking mine.
Always always take an enemy city or town even if it means losing a lot of armies ESPECIALLY right before the beginning of a new week. This takes gold and armies away from the enemy.
Chest decision is more complicated than it seems! There are 3 possibilities: 1) 2000g or 1500e. 2) 1500g or 1000e. 3) 1000g or 500e. In number 1 you pay 1.3 gold per 1 experience point. In number 2 you pay 1.5 gold per 1 experience. In number 3 you pay 2 gold per 1 experience. Always take gold from 3rd option. If you want experience, take it from 1st and maybe 2nd option, depending on situation.
Always have autosave enabled (it is by default) it saves your game every turn. When the game crashes, autosave saves the day. When loading, look for autosave.gmX files. X represents the number of human players. Be sure to give players correct spots before loading that game.
In multiplayer games always put spell book animation off and combat speed to fastest to save the time.
While it's not your turn in multiplayer, take time to check the thieves guild, look for the grail and think your plans.
When deciding between flagging a mine or taking a resource, you will always want to flag the mine because the resource will remain there in the same quantity, while a mine will bring you an extra day worth or resources.
Sanctuaries are places where no hero can attack you. What you do is land a good hero on one that is near a busy place, crossroads for example, and wait for an enemy. If the enemy is too strong you don't have to worry about attacking, but if it is a sure win, go!
Credits: Jeffrey Hilton (8 tips), Ryguy273 (1 tip), Nemo (1 tip) and Todd (1 tip).