On wall expandibility

[fabio]

Two issue found while playing on Fortress RMS:

• it's not possible to expand walls from the existing tower walls, I can start the wall building mode but I get the red wall and the message "you can't build wall here". However there are no terrain obstruction, indeed I can build wall not connected to existing towers at the same place;
• it should be possible to extend walls also if terminating wall towers are not available. In this case I simply can't start the wall building mode. I suppose this should also the vase when a tower is destroyed and I want to replace it.