Option 4: none of the above
Something similar to option 3 is how they’re supposed to work. But giving them the artillery tag is just wrong, and historically they were heavy infantry.
Instead of tagging them as “artillery”, they could create a “siege infantry” tag (or modify the siege unit tag) that partly cancels out the weaknesses of being heavy infantry.
It could have properties as follows:
- Light infantry and most artillery do x0.75 vs siege infantry
- Buildings do x0.5 vs siege infantry
- Cavalry does x1.25 vs siege infantry
- Culverins do x3 vs siege infantry
This tag could also be useful for other siege troops that aren’t quite proper artillery such as Battering Rams, Flamethrowers, and Arrow Knights.