If you put some cannons on top of a mountain their range will be the same as a cannon at the bottom of the mountain.

I set all the cannons on attack ground at their maximum range and took control of one of the cannons. Arching the shot just barely I can shoot farther then the supposed maximum range.

Here's an outside view to compare the angles. Note that all the guns except the one I was shooting are pointed flat.

Here's how much of an angle you need to shoot to the corner of the map. Looks like it's about 25 degrees.

I built a bertha down at ground level and had it shoot up at the hill and had the berthas on the hill shoot back at him on the ground. For some reason he had an easier time of hitting them than they had of hitting him. When it was over he had killed off two of them and damaged a third before being destroyed.

It's really stupid how artillery is implemented right now in the game. It would make a lot more sense if the shot power of the berthas was dropped a bit and their maximum range was actually the range of a 45 degree shot. The advantage of height should be taken into account too so you can shoot targets further off from on top of a mountain. Another way you could do it is to keep most of the speed of a bertha shot, but limit their maximum firing angle to something around 25-30 degrees. Again, make the max range actually the maximum range the cannon can shoot and increase the firing range appropriately when you are shooting to lower terrain.