When I started this game I did very badly, my first few cities just failed to survive beyond their first year. You know how it is, the false starts, the were creatures, the fact that you forgot something, the Dwarfs start to tantrum and it all goes to Hell.
Then the first REAL Fortress was finally founded. I loved it. I worked hard on the design, learning from past failures but also learning news things about how mass-dig bedrooms and where to put workshops and how to build a hospital and my first working wells. The floor designs were wonderful.
But my knowledge of defenses were crap. The first major invasion easily bypassed my walls, which had gaps, and my badly designed drawbridges and flanked my ill trained militia.
The Fortress after that was not as pretty but it was all about enclosing it within walls, drawbridges, traps, building platforms for the archers to rain bolts down on the enemy, siege machines with a clear line of fire across a natural river I built next to. I dug out channels, made sure all the bridges on the map could be lifted to make movement for invaders a issue, had a backdoor even made from a floodgate in case migrates appeared on the back edge of the Fortress.
If the First Fortress was Paris the Second Fortress was Fort Louisburg.
Yes, I learned about how to use windmills and built a above ground bathhouse, and a multi-level grand hall with a glass top so light poured down into the depths of the Fortress, and many other non-military engineering and if it hadn't developed some kind of save issue I would still be playing it.
My current Fortress, the one I think of as the Third Fortress, has a stone roof over the surface city, where a majority of the Dwarf live, and can be thought of as my mega-project but I still think of the Second Fortress as the one where I really learned how to design a properly protected Fortress but one that was also designed for daily life. Without the Second Fortress and the goal, the project of training me in the Dwarfs's Art of War, I doubt this MountainHome would have made it this far.
I think of my current Fortress as Rome. Powerful, wealthy, built on the knowledge of past cities, and unlikely to be brought down any time soon. It has already passed the ten year point of its founding.