After reading a large amount of threads about siege units, ways to improve them, and make the game generally more difficult(siege wise), I decided i should have a quick rundown of features mentioned, with some adjustments (for what i would consider, balance.)
All features was designed in order to make the game more interesting for turtles and/or defensive style players, If your worried about your Precious Dwarves, feel free to turn off sieges.
Siege timers:
-After a certain amount of time without meeting an engagement, either attacking, or a pre-emtive strike via dwarves, extra "special" Units would arrive on-Map.
-Times could include 5/10/15 (more?)minutes, could vary on enemy civilization strength/capability.
-Extra units shouldn't include necessarily more offensive units, but preferably units with more capabilities. (Mentioned Later)
-Extra siege units can be prevent from entering the battlefield by destroying the siege before it escalates.
Enemy Mobile Ballista:
-Can destroy Fortifications from range, up/down multiple Z levels.
-Can impale Marksdwarves, a Good hospital, and a sturdy set of armor should reduce chance of permanent injury Significantly. ("tiny" chance of death, wouldn't want years of dwarf training to just Vanish from a Ballista shot)
-Slow aiming (Difficult to target charging dwarfs)
-Max range is 20-35 squares? (Needs revision)
-Immune to most bolt damage
-extremely subceptable to melee damage.(Urist Mcsiegerape Slices the ballista "Bowstring" disabling Further fire., Urist McHammerTime Shatters the Ballista Bowframe, Bolts cannot be mounted anymore)
-Other units tend to guard the ballista, (Preferably using Formations if/when they're implemented)
-Reduced damage output compared to dwarven ballista, due to mobility implementation.
Cattle Catapult (Cattlepult)
-Will fire rotting corpses of animals and downed soldiers over Dwarven walls.(If non are available, the siege army will provide some from "off the map"
-Will generate small amounts of miasma and Disease (Bad thoughts) Regardless of being outdoors.
-Can be fired on any tile which is outside,Light and Above ground. regardless of "Ultra High" walls. (This reduces pathfinding a little)
-Cattlepult aims for any dwarf meeting those conditions, The cattle Will hit Wherever the Dwarf was standing at the time of cattlelaunch.
-Obviously, There is a delay between firing, and the cattle eventually landing.
-Dwarves working in "Cattlepulted" Conditions for large amounts of time may fall Unconcious temporarily.
-Corpses may contain large amounts of vermin and flies, which will be released shortly after the cattle lands.
Siege Builders/Siege Engineer:
-Should Definitely use Siege timers, Builders should be a last resort by Invaders.
-If a dwarven Fortress is simply Turtling, Builders will be able to Normally Overcome defenses and/or prepare for any other enventualities.
-For example, If a Mobile Ballista(Refer Upwards) has destroyed a wall fortification, a builder will be able to construct stairs up to the open space in the fortifications.
-This could include Building bridges over moats. Building their own fortifications To protect against marksdwarf fire.
-If pathfinding Says that even using a builders capabilities a fort is Incapable of gaining access, then a builder will construct a small camp/castle for the Invaders to reside in, during this "phase" no structure may be placed within 10 tiles of an Dwarven structure, Unlike Normal Siegebuild mode, Which is Anywhere, as long as there is "possible" entrance.
-Builders Have very little defensive capability, Will retreat Unless an entourage is provided
-Can detect traps, any trap within 5/10(Revision Needed) squares, Range may depend on attacking civlizations overall "strength"
Trap Detection(revision)
-Any trap discovered, either by a builder, or by activation by another hostile troop, any trap adjacent to that trap, is flagged to invaders, Along with any trap adjacent to that one, (etc)
-"flagged" traps will attempt to be disarmed by a Engineer, If it fails, the trap will activate as normal.
-If no engineer is available, All flagged traps will act as a Wall to invaders.
Catapult and ballista can be salvaged for wood and rope when destroyed.
Thats all ive got to say for now, i thought i had more, but i spent around an hour thinking all that up and trying to balance it in my head.