I have an island I want to attack where the defence view shows purple rings around the towers and available troops. What does that mean? I attacked it and got absolutely smashed so I guess it is upgrades of some type?
It’s an alliance upgrade from gold donations.
Almost correct, it is a divine buff placed on those troops/towers which makes them very powerful. You can click on your alliance, divine buffs and then check what each of the buffs does. E.g. Minotaurs do 30% more fire damage and I think their HP increases by 60%.
Word of caution: Do not attack an enemy with too many divine buffs unless you are ready to spend a lot of gems. Try after some time and maybe his divine buffs are gone