Upgrading your Citadel really does make a difference.

I rather enjoy when the enemy upgrades the creeps. Gives me more $$$

I was about to post a question on just this topic.

How useful are people finding the non-unit upgrades at the citadel?  I typically try to upgrade creep armor and ignore damage upgrades figuring the longer my creeps are lasting the less exp and gold my opponents are getting.

What do the other upgrades do and what is the point of getting them?

Typically I don't think I'd be upgrading creep damage much at all because I don't really want my creeps to be killing anything (that's my job) but more just staying alive as long as they can.

Also isn't it better to have your own creep wave smash themselves against the enemy players towers so the opponent doesn't get ANY exp and gold for the kills?

Well, citadel upgrades like skill points, still require strategic use.  Getting priests, angels, and catas all at once can often leave an xp starved opponent pushing up daisies.  DG's that tend to have to just walk up to towers and attack them will often get surprised pretty bad if you've been upgrading your tower armor and health.

As always, make decisions based on your opponents and the current state of the game.

I don't bother with creep armor upgrades anymore. It is pointless as demigod just blow them into the air in 1 sweep. I used to thought they could at least survive 1 more hit or something, but apparently not until like lv4~5 upgrade; by then, most demigod are already too powerful for it to matter.

I do get the attack for creeps though, it help killing tower where no demigod is present.

Tower health for the first rank is a must at the beginning of a game.  Gets 5 HP regen going on them, so it will regain what the creeps knock off of it.

The money upgrade is simple math.

(the cost of the upgrade) / (income provided) = (how long it takes to pay it back in seconds)

Just don't forget that the second level upgrade to 6, is from 4, it only adds 2, not 6.  And if you want, factor in your team mates into the number of seconds.  It will tell you when your team will have overall earned back the money you put in.

(how long it takes to pay it back in seconds) / (number of team mates) = (how long until the team gains benefit)

The damage upgrades are only really worth it after catapultasaurii.  30% for the first rank is a lot of tower damage being done there.  It is miniscule up til this point, so no point bothering to get it.

Armor, never.  Pointless as stated above, games don't last long enough for it to matter.

Demigod XP and Kill time reduction.  I never really buy these, and kill time reduction...  well if you are worried about it, you have bigger problems.  To defend the XP one though, I do buy the first rank sometimes, no real reason other than to out level my opponent a bit faster.  Never buy it with any kind of regularity though.

As for the extra minions.  I never really have any luck buying them all at once.  Priests actually DO make it take longer to kill creeps, most times it adds and extra swipe per minion they attack.  I'd say they are useful earlyish game, unless they have a TB or something.  But, if it's just single target or even a dumb rook who spams hammer slam on them and wastes his mana, I usually buy them.  That and you can't count out their heal that they throw on you from time to time.  Can really help early on if you get low on health.

The rest I try to buy all at once.  The first to 6k gold (assuming level 10 citadel) buys giants, everyone else buys the angels and pults.  You will have to coordinate with your team on this, it's alot of gold to spend all on your own.

That's basically what I do, it's not perfect if you ask a "pro" but it's how I roll and it hasn't failed me yet.  When I actually see someone with a better proven upgrade plan, I will go with theirs.

Sean is right, and really all the upgrades are simply math.  And most of the time, upgrades past level 1 or 2 don't add up.

The first money upgrade you should get ASAP.  The 2nd one is sometimes worth it.  The last one pretty much never is.

The catas and giants are worth it.. consider their cost compared to what they do.  However, your enemy will make much more money killing them than you paid for them.

But besides that, the towers upgrade is.. so so.  Creep dmg and armor only worth 1 or 2 levels.  after that the benefit usually isn't worthe the cost especially with the crepe survival upgrade. +45% dmg can sometimes be worth it though.

Early priests aren't always bad because they will heal you while you're attacking towers.  But.. much higher gold reward for the enemy.  Good on maps where you have both extra portals capped to do an early tower push.  Otherwise wait to buy priests+angels+catas at once.

Armour's the one I'm curious about. Gold, XP and tower health are a no brainer. Tower damage is pretty good, but not essential. Creep damage seems not very worthwhile, unless you're going on a concerted tower raid (and even then...). But armour I can see being potentially useful, but I rarely put money into until the late game. I see its use as not being just against demigods (although if it does mean the difference between one swing and two, or one AoE and two, that's still double the time). Rather, it helps a lot on medium to large maps with less than full slots, where a lot of the time creeps are fighting other creeps. The longer your creeps last against other creeps, the more waves will build up, meaning that when they hit the towers, there'll be enough for them to do some serious damage. But I'm still not convinced it adds up.

Demigod is a game of complex interactions between the mechanics and thus complex benefits and costs to your actions.  Every choice, in this case upgrades, has to consider more than just "will my DG be more powerful quickly enough if at all".  Even the choice to upgrade gold is not a matter of sacrificing the time it takes for your team to recoup the cost of the upgrade in gold earnings.  A less item dependent hero (e.g. idols/support Sedna) purchasing this will provide a good boon to the assassins on your team who need strong items to be effective without setting you back too much.

Let me give a more detailed example.  I dont think anyone has mentioned flag capping yet.  Upgrading creeps is a foolproof strategy to improve your control of the central flags which will, in turn, be worth many times the cost of the upgrades. Try playing the map "Brothers" without upgrading you creeps against a team that does.  You will lose.  That 1800g you got some extra mana or health with instead of priests just handed your opponents the artifact hut, nuetral creep portal, and celerirty flag.  They will use that portal to push down your outlying towers, teleport to their flags for instant front line support, and will continue to upgrade using the extra money they are earning.  You will be confined to your base which will become increasingly difficult to defend.  You might be able to counter this by turtling and building a late game regulus or rook, but you will probably need a bunch of defensive citadel upgrades to do it.  ...or you could just buy priests (and the others later).  One person spends a little on extra creeps or the entire team has to be able to hold flags against harder enemies and one person keeps a little gold.

To cover all cases and provide a prisoners delima type arguement: Assuming teams are of otherwise equivalent skill you have the following options: either upgrade creeps or not, they can either upgrade creeps or not. 

1) You upgrade creeps and they upgrade creeps - equal footing.

2) You upgrade and they dont - you have the above advantage (assuming your team can capitalize which is relatively easy).

3) If you dont upgrade and they dont - equal footing.

4) You dont, they do - disadvantage as described.

Hence you should always upgrade because the worst case is even chances and the best is an advantage.  The only real skill involved is choosing who will pay for the upgrade.

This kind of analyses can be applied to most of the upgrades with different results.  I believe the creep upgrades (HP, DMG, priest->giants) are the most universally helpful ones as you will eventually always need to push to win (except in DG kill limit mode).  Some others become rather worthless depending on your strategy.  (e.g. why would you buy tower damage if you bought priests and creep damage to keep the lines pushed).

If you really dont want to think it through, here is a guide:

*Always buy: Priests, lvl 1 creep damage

If you are getting outpushed, buy: Angels/Cats/Giants, lvl 2 Creep damage, Creep health

If your support character has early extra cash, buy: DG XP boosts, extra gold

If you are playing "forts"mode, buy: Building HP/regen

*Never buy: Death penalty, tower damage

Comments?

Tower damage may be worth more than you give it credit for: it adds splash, which lets it take down multiple units at once. . . which is ESPECIALLY important when a Soul Power + Spirits Oak comes crashing onto your towers pulling in +350 damage from his spirts.  Knock 5 of them out of the sky in one shot and all of a sudden his damage is a lot less scary

Similarly, if an enemy wave breaks through to your towers, taking them all down in 1-2 shots is better than spreading 5-6 over the whole group.

And for 500/600 gold?  Not too shabby!