friend of the multiple accounts is a very old subject, I advise to have as much 2 as to have more is crazy and stops being fun.

Plarium can not do anything because it is too complicated.

there are those who have a lot because they do not do anything else, others create accounts just to bother, etc.

The only thing I can tell you is patience that will get tired.

deutschland said:

(...) 3 players from different coalitions have started to raid my sieges. They do it at the same period of time with even the same forces (2 swordsmen)

Think why someone would want to break your sieges out of spite? Did you gave someone a reason for it?

There are people in this game who are a-holes about their sieges. When someone raids their siege they tell them "don't do it", others will scout as a warning, others will raid a city of person who raided their siege. Are you one of these people? Things like this don't just happen without a reason. 

deutschland said:

Now my question to Plarium is: what can I do in the case if I suspect multiple accounts? I know that Plarium does not handle such cases. Does this mean that you simply do not care? Is this a carte blanche for everyone to use multiple accounts?

Just a scenario:  Plarium staffs saw 10 accounts from the same IP address. Plarium ask them why so many accounts? They answer, in my household we play game together, everyone from the grandpa to the wifey to the kids and even our dogs.  If other player mess with one of us, they mess with all of us. That is why you see 10 unknown players send raids at you. Perfectly legal here.

My scenario above is just for fun, but it demonstrate how powerless plarium is concerning multiple accounts. Beside it take more resource to enforce something that potentially take away their earning like HEROES said above. Why drop a hammer on their feet?