Start production of siege units with infantry within healing range of a keep that has been upgraded with the healing tech from culture wing. The siege start to gain 2hp per second without being completed. I don’t see how it could affect anything, but it doesn’t seem right. I guess could maybe be exploited by placing a bunch of ram plans down around a keep, since you don’t even have to be at the plan you can just place it as ‘god’ and the keep starts working on healing the plan up to full hp for you to securely complete later.