Do you, personally, think it's a good idea to build a city on hills or tundra?
Terrain type is almost irrelevant for me in relatiely late game (about Corporation onwards). As long as the city can gow one pop point per day, then all is well. Swamp, tundra, even Glacier.
In very early game, I don't build on difficult terrain... but I pretend I did, and build around it. Later, I come back and backfil it. What is important is not just the terrain you build on, but the surrounding terrain and what you want the city for. Every city I build, I also know how many trade it will produce and how many shields it will make and what it will be doing (in general) thousands of years down the road. Shields get the most attention. If you build on a hill, you build in teams or don't build there... one build a mine, and the other makes a city. That way, you get the shields. I built on lots of hills in GOTM017, because of the small island layout. I think 4 of my first 8 cities were on hills.
About Walls:
Howies ignore walls. Furthermore, fortification behind city walls is does nothing for land battles that consider walls. Said another way, if you have walls, the wall bonus supercedes the fortification bonus. Similarly, the Fortress bonus superceedes the fortification bonus. Might as well sleep in a fortress.
A fortified vet Mech Inf on a Mountain behind Walls gets:
6
+3 Vet
+0 Walls (Howie ignores walls)
+3 Fortification (No wall used)
+12 Mountain
===
24d Total... against a Howie
6
+3 Vet
+12 Walls
+0 Fortification (Superceeded by Walls)
+12 Mountain
===
33d Total... against Armor, e.g.
A fortified vet Mech Inf on a Mountain in a Fortress gets:
6
+3 Vet
+6 Fortress
+0 Fortification <--- Not a misprint.
+12 Mountain
===
27d Total... against anything
A SLEEPING vet Mech Inf on a Mountain in a Fortress gets:
6
+3 Vet
+6 Fortress
+0 Fortification
+12 Mountain
===
27d Total... against anything
See? In a fortress, there is no point in Fortifying. Unless you will destroy the Fortress with a different unit that turn.