3) Don't be afraid to have a Fortress with lots of essence. They make really strong troops
More to the point, make sure that your primary troop training fortress(es) have lots of essence. Other fortresses can go anywhere, so once you have a fortress or two on a high essence spot AND you have enough troop-training capacity for your current needs, other high essence spots should probably become Conclaves or Towns (Conclaves get structures with per-essence bonuses, while Towns get lots of income bonuses; a high-essence Town enchanted with Propaganda and producing Wealth can be very good for your economy, and if you pick up the Mint of Ruvena at level-up and cast Arcane Forge, it gets even better).
Also don't be afraid to settle poor locations (e.g. 1/3/0 sites) if they have a strategic benefit, such as enabling you to block a mountain pass or deny the AI access to a peninsula. Just make sure you aren't losing anything better that can do most or all of the same thing.
Early game, two or three trained units (or wolves, dogs, or golems from tokens, or ophidians or butchermen from certain quests) make a decent enough field army for each of your champions without completely ruining your economy. Just try not to have more than two or three such armies, unless they are made up of the wolves, dogs, and golems you can find from loot, or the ophidians or butchermen you can get from certain quests. Strategic-level summons can also work well enough for your early game armies, if you have the mana flow to support them and don't need to make use of tactical spells.
Speaking of spells, avoid casting most damage spells in the early game. Burning Hands can do well enough, especially if the opponents are in a nice line, but things like Flame Dart aren't going to be effective without a mid- or high-level caster and lots of mana. Support spells like Haste and Slow are much better for the early game.
Before you settle an area, clear out the monsters. It's safer that way.
Turn on manual building placement. It lets you shape your cities to control the way your Zone of Control expands, which lets you grab resources or avoid waking monsters you can't deal with, and lets you block chokepoints with your cities. Auto-placement will result in roughly circular cities.
If you have the ability to build a lumber camp in a city, always place that down BEFORE building anything else, even if you later move it back in the build queue. If you don't do so, you could potentially lose the ability to ever place the lumber camp, depending on what shape your city becomes and which direction it grows in. Cities which are partially surrounded by forests (three or more forest tiles around the city) don't really need to worry about this, but cities with only one or two forest tiles nearby do.
Look at the local world resources which your cities can grab - if there are nearby Wild Grain tiles, that 1/5/0 tile next to the 4/2/0 tiles starts to look a lot more appealing, while if there are a bunch of Clay Pits nearby, the 6/1/0 tile may be just as good for you as the nearby 3/4/0. No world resources currently provide essence, though, and essence provides access to city enchantments, which are often very useful, and to the Cleric line of buildings; additionally, certain upgrades to structures only become available if the city has essence.
Essence is probably the most important of the city resources, as with the proper city enchantments it can provide food, production, research, or money, drastically improve locally trained troops, and a lack of it can restrict access to certain buildings. I wouldn't say that a 1/1/4 tile is better than a 4/4/1 tile, but I would say that in general a 2/4/2 or 4/2/2 is better than a 4/4/0, and a 3/3/3 is better than all of them. Remember that this is a general rule, though - forest and river access, what world resources you can get and how quickly you can get them (and how quickly you need them), and strategic considerations like pass blocking have a bearing on which tiles to pick, too.
If you have both Iron and Crystal in your starting area, Iron is the better resource to focus on developing in the early game, since Iron Mines are twice as productive as Crystal Mines, and the techs that improve your Iron Mines also improve your city production capacity and require the first of the food production techs. Beyond that, if you can manage to build the Ironworks wonder, you'll probably have enough metal income off of a single mine to provide for at least your early- and mid-game armies.
Early game research should focus mostly on the Civilization tree, unless you're pressed by a hostile AI faction. Knowledge, Civics, Trading, and the lines leading to Construction and Economics are very good for improving your empire's infrastructure, and the Economics line also includes a few techs that boost the size of your trained units. Don't blindly focus on getting these, though. Leatherworking and perhaps Weapons in the Warfare tree are also good to get early, and if you have horses or wargs you might consider picking up the Mounted Warfare line as well. Early Magic technology is only really good if you've got lots of crystal or want to build on some shards.