I think the first gold upgrade is always a good investment, doesn't mean a AUTO win in the game of course. But in Fortress I think that is the most important and coupled with that the building Health Regen to make sure your defenses and fortresses can't be wittled down over the course of the game.

The first gold upgrade is pretty well a must-get.  It's absolutely phenominal and if you can't afford it because you're being pushed too hard chances are you're going to lose anyways.  After that, they're usually not worth it.

the upgrades past lvl 1 gold are worth it when u have mor eplayers than 4

only two conditions make the second and third gold upgrades worthwhile. the first condition is a 5 player game. the second condition is a game with the gold rate set to fast, this makes the time between gold ticks speed up. yep, it will give gold more often than once per second on fast setting. makes those upgrades worth alot more. 

generally though the first one is great, the other ones are a waste. 

regarding whether or not playing Fortress mode makes a difference...i'd say it depends

if you're on a small fortress map that has easily accessible forts than its probably not worth it and you should just rush the fort. buy Idols (if a general) or just health potions so you can whack at the forts for longer periods of time. 

if its a large map with forts way in the back, its almost as if you're playing conquest (just with fewer flags on the map), so yeah, i'd say gold upgrade is very important for that type of map. 

I agree 100% on the 1st gold upgrade, but even that depends on map... I personally wouldn't upgrade further than lev 1, but it all depends on who you are up against.