Too easy to use artillery with their ability to see through obstacles

[BornGinger]

One thing in connection to artillery and forts I don't like very much and which could be changed is that terrain doesn't obstruct their view and thus their ability to shoot.

It would have been better if it was required for another friendly unit to have clear sight of the enemy and thus report the enemy's position back to the artillery/fort behind a forest or hills to make them able to shoot at the enemy.

[Horseman]

You do need line of sight to a unit to allow your artillery to shoot it?

And don't forget Artillery spotters existed. People who's very job was to go put and spot targets for the artillery to shoot.

[Blade0]

Yeah... when an artillery already sees its target, then it is high time to leave everything and run for your lives!
Artillery were used to fire at an area that was pre-planned and (hopefully) agreed with the advancing infantry, or given coordinates "on-the-fly" by spotters through radio when it was well-spread enough.
In the 1st WW British and French has successfully developed a "creeping barrage" that went forward in a pre-set pace salvo by salvo - the friendly infantry had to advance just behind it to maximize the effect. In WW2 radio was way more common and compact, and they have begun to use radar to acquire target coordinates at sea.

Now direct-fire guns (ATG-s or tank guns) are different - they don't fire on a high trajectory, and usually deliver a higher velocity armor-piercing round. They are way more precise, but their range is limited compared to artillery - an ATG or tank gun is rarely fired above 500-800m, while a 150mm artillery can be used around 10km (max range depends on barrel length).