Feel like Parson Defending Gobwin Knob in Book 1, except with more mechanical traps and customizable archetecture (and the ability to choose where in the world's landscape/environment to put your fortress)! Sadly, the game does not have Caster units (anyway Dwarves have zero magic units except maybe "runemasters" if they even have that ... and but the mechanical traps actually are reasonable enough compensation).
If this game was Modded with Casters it would be a reasonable approximation of Erfworld ...Slaves to Armok: God of Blood Chapter II: Dwarf Fortress
Please note this game comes in pixel graphics
so you don't go crazy looking at . and / and @ and n and ...
the game also has a 3D visualizer
so you can admire your architecture.
Build your fortress from scratch, both above and below ground in a truly 3-dimensional environment (has many z-axis layers). Manage the civilians' work and morale (this is actually the annoying part ... this game is in alpha after all, the civilians are too dumb to live so you have to carefully queue their tasks and keep everything running smoothly or else they are interrupted and lose all of their queued jobs ... their morale system is well-done, though, unlike their work system).
Use levers, water pipes (fluid logic), gears, touchplates (unit steps on it, triggers trap or whatever), etc. to create Dwarfputers to open doors, cage traps and other traps, retract bridges, and feel like a real Dungeon Master!
Create subterranean Greenhouses (with shafts to let the light in), above-ground greenhouses, vertical farming greenhouses, skyscrapers, and more!
Yes the game comes with siege equipment like Ballistas.
Use windmills or waterwheels and manage your power supply to run your gadgets!
Become an Evil Supervillan by creating massive hammers on the tops of your walls that are hundreds of feet long with a hammerhead tens (or hundreds if you have the resources) of hexes wide, creating a self-destruct lever which triggers the pumping of water or lava into your fortress(es) in case of defeat to kill invading forces, or creating an orbital magma cannon (giant building that has a mushroom-shape, that has a top that covers the entire map and drops lava on your enemies).
Magama cannons are also possible, but only fire a few feet, dropping magma from orbit though makes the range as far as you want. Giant chasms can also be constructed, so you could copy Sauron's Dark tower with this system if you wanted to (you can even construct the building out of obsidian by playing with cooling magma ... so that it looks appropriately ominous).
Defend yourself against Goblins and Orcs (and Humans and Elves occasionally or if you anger them). If your above-ground castle falls, retreat to the caverns below and continue the fight against the siege.
Harvest hundreds of different resources (rocks/ors, plants, and wood). Combine some to make explosives, etc.
Each Dwarf has a chance to create one Artifact weapon/armor/tool/clothing/statue/furniture in his lifetime ... over time you can horde these weapons for war (and furnishings to vastly improve your Dwarves' morale) ...
Experience a combat system that includes bones breaking (and limbs being chopped off ... permanently), which is so much cooler than Nethack. Also, there are seasons and in cold places in Winter the water freezes over.
Install sprinkler systems to put out fires!
I built a fort with a fire sprinkler system once. I put cisterns full of water over my central stairwell and my booze stockpile, and if I do it in the future I'll put one over the hospital and one in any area where dwarves work near magma. I made the fire alarm levers out of realgar. To prevent any flooding disasters, I connected my sprinklers to closed cisterns that were filled with just enough water and then sealed (rather than a constant supply of water), and made sure to build adequate drains for all sprinklered areas.
Oh and I forgot to mention that you can tame animals. And capture Megabeasts - ex: Hydras - (which you probably can't tame) to unleash upon invading armies before you send out your melee troops.
As you can tell, this is an engineering RTS base/dungeon defense game (and you can successfully genocide the other races/monsters attacking you so this isn't endless and unchanging). Very, very original. There is also a "dungeon crawling mode" but honestly Nethack has more content even though my engineering geekiness likes that Dwarf Fortress could go beyond Nethack in terms of epic flooding chambers of lava, crumbling bridges (because they are being immersed in lava), as well as your Hero worrying about losing limbs ... etc. It's shortcomings are lack of diplomacy options, stupid civilian AI (though the enemy sieges are difficult but only because of the numbers of enemies not particularly clever tactics - fortunately the enemy AI and civilian AIs are *dumb* but not buggy), and lack of a research system (or else it would be both an awesome engineering experience AND have the features of Civilization IV).