Does anyone actually like the idea of separate siege and ranged classes ? Especially once you hit Renaissance I just kind of feel like you would be equally likely to want to use artillery against enemy units as enemy fortifications.
The Big Change in warfare took place with the introduction of effective gunpowder weapons in all Classes of troops, and it tended to be towards increased Simplification of Types of Units.
So, the Anti-Wall class of equipment and techniques ('Siege') started out very specialized: Battering Rams, Towers, Sambuca, Catapelta, Ballistae, Trebuchaes were all very specialized for dealing with large, immobile targets. Alexander and his father used Catapults against a field foe exactly twice (once each) - both in very special situations - and the Roman Imperial Army had light Ballistae that could accompany troops into the field, but appear to have had mostly a Morale Effect (something about a three foot long iron dart whistling past your head makes people Nervous) rather than a purely Military Effect.
The first Bombards were the same: they took hours to reload, so were totally useless against anything or anyone not nailed in place, but they quite simply made all previous stone fortifications Obsolete: no Medieval or Classical vertical 'curtain walls or towers remained vertical for long once a Bombard set up in front of them.
Once gunpowder artillery became reasonably reliable (early Bombards had a distressing and too-frequent tendency to Catastrophically Fail by blowing up in their owners' faces when fired) they became light enough to move and be useful on the battlefield. Early (16th - 17th century) artillery came in a bewildering variety of sizes, but quickly became classified as:
Light or Battalion Guns firing projectiles weighing from 1.5 to about 4 pounds
'Battery' (Field) Guns and Howitzers firing projectiles weighing up to about 12 pounds (twice that for low-velocity howitzers)
Siege Guns, the most effective of which fired 24 - 32 pound projectiles BUT, with effort, could still be moved around the battlefield. Notably, Marlborough's artillery commander, Brigadier Blood (great, great name for an artilleryman, by the way!) massed and moved up 22 24 pounders onto the battlefield at Ramillies and literally blew a hole right through the middle of the French Army. Frederick the Great used 24 pounders to support his infantry attack at Leuthen, and referred to them as his "Dogs of War"
Which means, in a nutshell, that Siege Equipment was no longer limited to Siege actions once you had 'Field Cannon'.
There was a tendency, especially in the late 17th - early 18th century, to mass all the Siege Guns and digging equipment into a Siege Train, which is why I included it in my Suggestions, but it really lost a lot of its meaning in less than 100 years, and by the 19th century and especially the 20th century (late Industrial through Atomic Eras) Artillery did it all: weapons up to Soviet 'High Power' 203mm howitzers could blast fortifications from a distance, flatten field defenses and units, or, in extreme cases, be brought up and fire Directly at individual bunkers from a couple hundred meters away (which the US Army in 1944 also did with 155mm 'Long Tom' heavy cannon). No separate units for 'siege' work are required
If asked (which you didn't, but what are Forums for, Eh? here's my 'line-up' of the Ranged - Siege Lines:
Siege (All Support Units - no separate Combat Factors if Melee Attacked)
Battering Ram (Ancient)
Siege Tower (Ancient - Classical)
Catapult (Classical)
Bombard (Renaissance)
Ranged
Slinger (Starting Unit)
Archer (Ancient)
Crossbowman Medieval)
Field Cannon (late Renaissance - Industrial)
NOTE: also about this time, an Incremental Upgrade or Promotion for your Melee (Gunpowder) units of 'Battalion Guns' adding + Melee Factors
Artillery (Modern Era)
Rocket Artillery Systems (Information Era) (Yes I know 'rocket artillery' dates 'way back and was used extensively in WWII/Atomic Era, but the long-range accurate rocket/missile systems only became worthwhile with computers to calculate their guidance in the late 1970s - early 1980s: I know because I was teaching those systems at the US Army Artillery School at Fort Sill in 1980!)