Blue border is your stronghold. Orange is another clan's stronghold. As for unseen costs, in not exactly sure what you mean.

Your first question  was answered.

 I think you will find their are unexpected costs, and potentially hidden costs as well, it all depends on your point of view.

Sometime after I started playing I thought I  would have my own strongold (Ive since seen it done).

I thought all I had to do was get all the materials and the scrolls and build up my influence and that would be it.

I would have all the benefits of having a stronghold.

Then I found out that the benefits are not so many,and getting a stronghold is only the first step. There is almost no end to the resources that you need to achieve anything inside.

Once you have a stronghold it will add significantly to your influence and you will find the competition target levels increase, and you are likely to be drawn against opposition who also has a strongold.

If you have lots of eager clan members you wont find it so tough, otherwise it might be a grind.

Bonjour , comment  augmenter le  nombre de  renfort pendant  odin , la nous sommes a 265 millions  possible , le probléme c'est que tout le monde ne peut y mettre des  troupes en renfort . 

There are no unseen costs.