Lusotannan Observations

[bovi]

'Simple' allows weapon upgrades while 'archery' does not. It's up to the team to figure out which weapon types or units deserve smithy bonuses.

Oh, yes this is right, I didn't think that was the question.
Archery does allow bonuses, we just don't give them (as increased attack for missile units is bad for balance due to their hardcoded 100% lethality).
Simple/blade weapons are upgraded by the blacksmith.
Other is elephants mostly, no upgrades allowed.
Siege is for reform units. We add a penalty to these "weapons" on a building in order to make them unrecruitable, until we add another building with a bonus to cancel it out at the point of reform.

Technically you could upgrade other as well, but the weapon type display string is shared with the siege one, saying "Cannot be upgraded". I'd want to change this to "Reform unit" for siege if it was only separate strings for the two, but the way it is the elephants would all get that too .

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I thought the question was about why certain javelin-toting guys have archery and other similar ones have simple. I don't know this, but Blacksnail's explanation makes some sense.