It's important to know what to focus on with upgrades. So what should you focus on?
Walls can be upgraded instantly, so walls can be upgraded regularly as soon as you have spare money. Calculate how much gold is needed for everything else first, then spend all the excess gold on walls. Upgrade the outer walls before the inner ones, since the new updates just need you to destroy as many houses as possible to win. The castle is still the most important, but less than before - besides, usually if they can breach the outer walls and the middle walls, you've lost anyway.
Of all the towers, the higher the tier, the better the value. A tower might seem strong early on, but as you level up and your opponents get tougher, it'll be forever before an early game tower (upgraded, even), to beat something with lots of health, like an Ent. It all comes down to maths:
A Tower (lvl 9) has 860 health and 56 DPS (damage per second), attack one target at a time. Towers seem inexpensive comparing to its higher level brothers, but stacking it all up it is actually quite costly to get Towers strong enough in mid-late game. To get a Tower to lvl 9, you need:
150 + 750 + 4,000 + 15,000 + 35,000 + 45,000 + 150,000 + 300,000 + 300,000 + 600,000
= 1,149,000 (gold)
and the time it takes is:
5'' + 1' + 5' + 30' + 2h + 8h + 1 day + 2 days + 3 days
= 6 days 10 hours 36 minutes 5 seconds
=> build the high tier towers as soon as possible and upgrade them first, always. Upgrade the weak towers for as long as you need them, and the moment you have the strong towers, switch. This is the same with air towers (did I mention if there are Eagles, Towers can't defend while Ziggurats can?).
=> upgrade the splash damage towers first, single target towers second.
For example, you have Tower of Fire, Ziggurat, Storm Spike and Cannons. Upgrade ToF first, then all Zigs, then Cannons, then Storm Spike - divide by rank first, then upgrade the splash before the single-target if they are of the same rank.
Spells are easy - it's instant, and it's free. You upgrade a spell when you can, since the weird ingredient to upgrade one has nothing to do with the other. What matters is the mana to build them.
Cost 1 mana, deals 350 damage to a single building at lvl 1.
Cost 2 mana, deals 175 damage to a square of 25 units and 1 unit more in the middle of each of the square's side, making a "circle" of 29 units.
29/4 = 7.25
<=> 8 towers (because even if you hit a single square it would count as the tower being attacked)
=> 8 x 75 = 600 (damage)
But no one does that. A tower at the very least is 1 square away from each other, either because it is surrounded by walls (bastion) or by empty space (to block, as all buildings get a 1 unit circumference as non-deployable area). Furthermore, clumping a tower in one spot will leave other area undefended.
At most, Fiery Meteor can damage 2 towers, and hopefully a bit of walls and some non-essential buildings nearby.
It's more important to take out the strong towers and the splash damage towers first. Only use Meteor when you're facing a big bunch of walls and can take down a few towers while you help your ground troops get through. Remember, a spread out pack of 6 Towers is not remotely as dangerous as 2 Ziggurats of comparable damage. Kill the Tower of Death as soon as you see it!
=> Focus Celestial Lightning and only save up a few Meteors as a situational tool.
Troops is easy - again, showing why the game is biased for attackers - upgrade the Headquartes for more troops, and you'll always have too much food to spare anyway.
For this part, there's almost nothing to say - fire at will
My other Throne Rush posts:
* Throne Rush Guide for Beginners - Detailed Guide on Playing without Paying