You do not have to wear or use castle skins as an honorific in order for the bonuses to be applied.  Similarly achievement bonuses stay permanenrly in place once earned.

It is otherwise with hero skins.  Your hero only gets the bonuses of the particular skin which s/he is currently wearing.

There is an exception in respect of castle skins.  To gain the bonuses of a Bastion of the Beast or Dragon's Citadel item the item must be applied and the skin worn. 

You, and/or your interlocutor, can check this for yourself.  Click on Palace - Military - Statistics - General Info where the bonuses for castle skins appear and remain constant whether you put on one castle skin or change to another;  and click on Palace - Hero where the hero skin bonuses are shown and can be seen to change as particular hero skins are put on or taken off.

Thank you very much and i will check. i dont know why i didnt think of that.

Just to make it clear these town skins you need to wear/activate to get the bonus ...

bastion of the beast (John mentioned)

dragons citadel (John mentioned)

Raven's rest (yes I still have some)

Home of the Aiser (yes I still have some)

and there is one other with same propertiees of Home of the Aiser that I just used my last one ...

blazebo said 

"and there is one other with same propertiees of Home of the Aiser that I just used my last one ... "

it's called "the cradle"

Cradle or warriors fortress both skins have the same bonus as the home of the aesir.