The developers have not really emphasized this but based on the new streams of video we got yesterday, I noticed there's differences in elevation on the continent now! Yes we've always had hill terrain and mountain terrain but if you look at the E3 gameplay there are actual gradual changes in elevation from the beach heads up north to the actual city itself. To the south of the capital, you also see a gradual rise in elevation going from the west and rising towards the east as you come closer to the mountains.
Example 1: note the small lake and how the river feeding it slopes and ends below the hill. Also further north you'll see that at the point the grass ends, it declines further into the beach area
Example 2: This is a modest example since the bluff is only one tile but illustrates how GRADUAL the change in elevation is. In Civ 5 this would be illustrated by a round mound that sloped back down so the next tile would end in 0ft of elevation.
Example 3: In the last pic you could argue it's a new graphic but it doesn't illustrate net elevation, but this angle should. Note again the lake sloping. Also notice the green hill declines into the flat green lands which connects to the beach. But if you look at the city tile it sits on only flat land not a hill tile. Also look at the pyramids in the north. They sit well below the rest of the city. This is despite the fact that Wonders in Civ6 are graphically massive.
Example 4: This is a grab at the beginning just to illustrate the city starts on flat land yet it sits higher then the pyramids.
Example 5: This is the military encampment south of the capital. Note that it sits much higher then the other hill that sits to the north of it. Also down at the bottom you can see the he line slopes down towards the plantation area. That area sits well below the encampment. You can't say it's just that the encampment on a hill because look at the marsh east of the encampment it's visibly higher than the marsh in the top left corner of the pic. This illustrates that there's something other than the specific terrain tile that dictates height.
Example 6: Lastly, follow the path of the left arrow, it slopes down into the river below. And despite being flat land, slopes down into the same military encampment further north. Look at the ridge line near the river and forests. Notice the ridge is higher than the forest trees. As if it's a plateau. Also noticed the brown tree line is much lower than the flat lands by the barbarian village. Lastly, this is inarguable. The calvary is standing in flat land and is visibly higher than the marshes below. To further underscore that it's elevation. He's visibly higher than the military encampment as well.
Either it's a bit of distorted perspective on my part, or its elevation or there is a curvature of the earth
Anyway this could add an interesting turn in war as you have a new variable to add to your tactics as you choose battle ground. It also means you can settle a city at the top of an incline and force invaders to have to battle uphill just to get to you. You can also set a line of defense in which most your army is elevated above your incoming enemy, and it isn't abruptly changing from tile to tile.
It will also be interesting to see how modders both adjust and take advantage of this when making maps.
Imagine recreating battle scenarios with elevation a part of the equation.
***EDIT I do have to say that it's speculation and an assumption that Firaxis will make elevation difference a factor in battle. It would be an opportunity lost if they did not. Then again they could scratch that if they fear the AI or new users wouldnt know how to use a feature like that...
****EDIT to illustrate screen shots not present previously
All screenshots reference this video
https://youtu.be/1_0CfBYFJh0
Example 1: note the small lake and how the river feeding it slopes and ends below the hill. Also further north you'll see that at the point the grass ends, it declines further into the beach area
Spoiler :

Example 2: This is a modest example since the bluff is only one tile but illustrates how GRADUAL the change in elevation is. In Civ 5 this would be illustrated by a round mound that sloped back down so the next tile would end in 0ft of elevation.
Spoiler :

Example 3: In the last pic you could argue it's a new graphic but it doesn't illustrate net elevation, but this angle should. Note again the lake sloping. Also notice the green hill declines into the flat green lands which connects to the beach. But if you look at the city tile it sits on only flat land not a hill tile. Also look at the pyramids in the north. They sit well below the rest of the city. This is despite the fact that Wonders in Civ6 are graphically massive.
Spoiler :

Example 4: This is a grab at the beginning just to illustrate the city starts on flat land yet it sits higher then the pyramids.
Spoiler :

Example 5: This is the military encampment south of the capital. Note that it sits much higher then the other hill that sits to the north of it. Also down at the bottom you can see the he line slopes down towards the plantation area. That area sits well below the encampment. You can't say it's just that the encampment on a hill because look at the marsh east of the encampment it's visibly higher than the marsh in the top left corner of the pic. This illustrates that there's something other than the specific terrain tile that dictates height.
Spoiler :

Example 6: Lastly, follow the path of the left arrow, it slopes down into the river below. And despite being flat land, slopes down into the same military encampment further north. Look at the ridge line near the river and forests. Notice the ridge is higher than the forest trees. As if it's a plateau. Also noticed the brown tree line is much lower than the flat lands by the barbarian village. Lastly, this is inarguable. The calvary is standing in flat land and is visibly higher than the marshes below. To further underscore that it's elevation. He's visibly higher than the military encampment as well.
Spoiler :

Either it's a bit of distorted perspective on my part, or its elevation or there is a curvature of the earth
Anyway this could add an interesting turn in war as you have a new variable to add to your tactics as you choose battle ground. It also means you can settle a city at the top of an incline and force invaders to have to battle uphill just to get to you. You can also set a line of defense in which most your army is elevated above your incoming enemy, and it isn't abruptly changing from tile to tile.
It will also be interesting to see how modders both adjust and take advantage of this when making maps.
Imagine recreating battle scenarios with elevation a part of the equation.
***EDIT I do have to say that it's speculation and an assumption that Firaxis will make elevation difference a factor in battle. It would be an opportunity lost if they did not. Then again they could scratch that if they fear the AI or new users wouldnt know how to use a feature like that...
****EDIT to illustrate screen shots not present previously
All screenshots reference this video
https://youtu.be/1_0CfBYFJh0