The Ryzom engine doesnt seem to be friendly towards adding new objects ingame.. all the temporary things I have seen seem to be placed and removed by patching (Christmas trees/presents, Halloween cauldrons,easter eggs..) so the ability to "build" objects may be a problem

However, there are possible ways to work around this. There could be two types of buildable siege equipment - emplaced, and manned.
Emplaced is things like minefields, barricades, autonomous weapons, possibly buffing/healing stations.. These would probably have to be treated as an extension of the Outpost guards and I really dont know how it would be possible to implement them.
Manned equipment is pretty easy though. This is stuff like Artillery (real artillery, not OP guards ), mortar and machine gun pits, aid stations.. let your imagination go

What you do is have a craftable weapon, that weighs so much the wielder cannot move, with a 3d model that is large enough to obscure the homin carrying it. Require a couple of packers worth of mats to produce each item. Leave enough bulk free on the wielder, that other homins can give them ammunition. Ammo would be like existing launcher/autolauncher ammo, only more so, so the gunner would need a support crew crafting ammo and passing it to them.
These artillery and mortars should not be able to hit OP guards - only players. they should have a range of maybe 100-150 m, requiring a forward observer to feed targets to the gunner (or targeting macros

Defending guilds with access to their guild inventory do have an advantage here - it may not be a big one though if most major guilds are struggling for GH space anyway

There are still several issues to be worked out, this is intended to get people thinking
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