so then, if you destroy another player's structure, then you give Enmity for that action?
(just as you would for taking plunder?)
Destroying another player's structure IS plunder.
After a successful raid against a province, you may plunder one site on the province board but you may only plunder a site whose plunder value is less than or equal to the number of successes you rolled. Place enmity tokens equal to the plunder value on the site.
Each built structure is a site, and has a plunder value printed on it.