Ooookay .... but I would like to construct ACTUAL fortresses as well.
they are used all the time ... smaller than castles, little to no civilian population ... an oversized military outpost with thick walls and (in later eras) lots of cannonfire.
I suppose this brings me back to wishing Keeps and Fortresses had turrets, and that if you built a city attached to a Fort it could upgrade into a Keep.
However, there are fortresses of all sizes ... and I would like to see defensive networks be buildable by People who are into that sort of thing, and have enough stone for it to be feasible. Say, a series of Fortresses and Walls, with the occasional Gate, with roads from the forts and roads to the cities. A fort with walls on either side, and the gate going through the fort ... would be a VERY well defended stretch of land 
Potentially a Marginot Line-esque defensive line (but with archers instead of cannons), and lets not forget the Land-Mine-esque magical traps. (Marginot Line *was* effective, actually, and the Ardennes offensive would of never worked if the Allies had coordinated, instead of arrogantly upholding their Rivalries and such. The French had more Tanks than the Germans!! and most of them were used in full Armored collumns. The strength of the Germans was actually in their Coordination and Defenses. The West simply couldn't afford (or didnt want to afford) attacking the German line due to the massive fields of German mines. Mines were a relatively new invention of warfare, as was the hollowed-explosive. The germans were the first to use these in the opening years of WWII (or at least the opening round) ... they used Hollow Explosives to destroy the "indestructable" turrets at the Belgian Fort Eben Emael. That being said, even with the first usages of Airborne Infantry, Gliders, and hollow explosives ... the Germans were in some ways technologically and industrially behind the allies in the opening curtain of the war. It was only after the capture of France that Germany had a much better production of tanks and planes. Of course, that started to turn sour with the Allied Bombings from Britain ... but that is aside the point.
The point is that a Marginot Line could of worked in a WWII setting, and in fact the Pre-WWII Nazi Germany spend MOST of its Military Budget on DEFENSES!!!! lines of bunkers with similar function as the marginot, as well as vast fields of anti-personel and anti-tank mines. Also, Poland did have some tanks and planes, and the Polish Cavalry was simply a way to get the Polish elite infantry where they needed to go quickly. They weren't charging tanks on horseback, they were riding to Point B, and attacking/killing the German supply train trying to get from Point A (fuel) to point C (stalled line of tanks). In fact, the Polish military devised effective strategies for attacking/destroying tanks that were stalled in such a manner. Generally all trapped onto a narrow road-way.
That being said, I think we should have the option to build up Mundane Defenses and Fortifications (as well as raise up mountains as stated earlier ofc) ... such as walls, roads, and Fortresses.