I try to go for a combination of security and visual grandeur when building my entrance. My favorite, best entrance was that of Stonehall III, one of my longer-lived and more successful fortresses built into the side of a tall, foreboding mountain whose name has been lost to time.
Basing the entrance partly off of the gates of Ironforge, I wanted something big and recognizable, something my ranged and artillery element could fire from with impunity and looked good doing it. The result was a 8x by 6x by 4z cubbyhole carved into the mountain, ringed with fortifications overlooking a reatracing drawbridge that covered the ramp down into the entrance hall proper.
The walls at ground-level were engraved by my finest(original) engravers, and immediately above that, an unbroken line of fortifications provided an unrestricted killzone against any foe foolish enough to venture so close. The z-level above that was ringed with more engravings, and the cieling level featured a small platform with golden statues and a door leading into the upper level of the fortress. The door was meant to allow any flying adventurer quick access should the fortress fall, and served as a dumping point for prisoners. Anyone that survived the fall was free to limp to freedom.
Later, when I managed to start a dragon breeding program, I set up a flame bunker in the passage underneath the retracting bridge for added defense. After that was an area I called the Hall of Battle, a large, weaponized, 2z high entrance hall littered with traps my melee fighters gathered in to meet anything that got past my defenses. Here dwarven warriors did battle amidst pillars of stone engraved with feats of heroism, terrible beasts, and fine cuisine, capped by carefully selected statues of great figures form dwarven history to watch over them as they did battle. above them at ceiling level was another ring of fortifications, from which a waiting crossbow unit stood by to support my fighters from.
This defenses of this grand entrance never failed, mostly because I was vigiliant about the retracting bridge. The one I didn't activate the bridge in time, the dragon bunker managed to incinerate the attacker, a giant, before any harm was done. As is often the case, the bane of Stonehall III came from below.
This is a rendition of that glorious entrance as I remember it. Sadly, I have lost this fortress to time and file cullings, and cannot provide a more detailed image with one of the various visualizer tools. I try to replicate the design as much as I can, but I rarely find the proper conditions for a perfect copy.