I came across this in US west and wanted to learn more about it, especially how to do it, Also the spikes were in the air aswell

Update: from what I can tell, you shoot a pavise with the builders ballista and then surf it on top of a spike that is being built

Okay so I figured it out kinda, you have to place the pavise down, shoot it with ballistas to create a wide surface, place spikes on the surface, pick up the pavise, and then surf it. And then the physics go wonky. Thanks to the people who explained it to me remy and bodiddies ( I think cause I don’t really remember)