The first gold upgrade is very nice, as it pays for itself in about 5 minutes. Higher ranks, however, aren't very good as the bonus isn't as large. Getting this early is a great way to out-gear your opponent.

The xp upgrade is great to outlevel your opponent early, especially if there is an xp flag that you can maintain control of. But I put the higher priority on the gold upgrade, as higher level items will let you defeat enemy DG even if they outlevel you. But if your enemy gets priests, then definitely go for the xp upgrade. Which actually leads to the next point:

As a rule you should generally avoid getting priests unless you have the warscore/gold to quickly upgrade to at least catapults. They give nearly twice the amount of gold and xp and are usually more of a detriment than a benefit.

Finally, there are other useful upgrades as well. The building health upgrade greatly extends the lifespan of your towers and is probably the first upgrade you should get in a fortress match, especially since it's cheap.

thats what i thought so far... but i wasn´t sure.

what to do with the other updates?

First level of building damage is decent because it gives your towers AOE attacks.

My general rules, and this can change depending on my opponents tactics:

1. Gold Upgrade 1. The sooner you get it, the more gold you'll have collectively across the duration of the match.
2. XP Upgrade 1. Usually, I only get this if there is an XP flag, as mentioned above. This will increase the XP gap between yourself and your opponents enough to help decide the outcome of the game.
3. Building Regen 1. If your enemies are pushing against your towers right from the start, I'll grab this first. It's usually a good idea to wait a while, as most games take about 3-4 minutes befores towers face any real dangers.
4. I avoid the Death duration decrease, as basically my intention is to not die.
5. I avoid getting the priest upgrade until my team has a clear advantage on the field, thus increasing our ability to push, or until the enemy purchases their upgrade.
6. I save the DMG and Armour upgrades for the creeps until after Giants have been purchased, or if it's clear the enemy has purchased theres. The increases are marginal at best, and really need significant numbers on the field to make a noticeable difference.

I don't think it's a good idea to make a set pattern... do what you think is best in your given situation.

For example, if the level of play is high (most people are good), buy gold then xp because your teammates know what they're doing and will really benefit.  Otherwise, focus more on outgearing them and upgrading towers so your teammates have a place to hide and the enemy can't make progress as easily everywhere you are not.

Have people used the building damage/health upgrades above 1 much? Is it worth investing in those?

I once played a 3vs3 on leviathan where it seemed like the opponents bought a lot of armor and damage to their creeps and then upgraded to priests. It was crazy. Couldnt kill more than 2 waves of creeps before my own health had almost disappeared. It really felt stupid because we were really afraid of their creeps in that game.

Has anyone else tried out this tactic?

Just lost a game where the enemy had fully upgraded their troops - you could really feel the punishment.

It gives you 4 gold per second. The second upgrade 6, the third 8.

In a 4v4 game (on the big round 5v5 map) our team got totally owned yesterday, but we won in the end, because they team ganked all the time, but we had higher levels and we got priests at the right time, so basicly not adapting their strategy got them killed in the end.

I'd say on big maps you can get the priests earlier, especially if there is alot of team ganking (which there usually is, especially with towers=strong). The team ganking lets like atleast 2 of 4 lanes of you priests push thorugh, if you keep the enemies occupied.

Gold I - Like many wrote before, do it as soon as possible. Your team (and even you) will get more gold than paid in no time. The other upgrades are imho to expensive for that fast game. Maybe the 2nd in a 5vs5 game I would buy, when I don't need the gold for items or I got many kills (and so the most gold, too).

Tower Upgrade I's - They are cheap and give them skills they don't have before: HP regeneration and AOE. I would get them early and nearly always. The other upgrades don't help to keep towers up, because they are normally attacked by 2+ heroes or catapults, where more hp regeneration or damage doesn't help. At least with those low values. (My experience).

Death Upgrade - You should follow the number one rule: Don't die!

Creeps - Wait till you can attack the base or the enemies bought it. Damage and HP upgrades are very expensive for their low bonus, but you have to have stronger creeps than the enemies if you want to push instead of defending against creeps.