Making a tunnel for easy human access between two nations, where it prior was an inaccessible mountain range.. sounds like something that you really need to request permission from the local monarch (who in turn likely makes a deal with the neighbouring one)? Unless you want a huge diplomatic incident?
Because facilitating army movement on a large scale, enabling the ability for any invasion force (either direction) to use a border that was considered safe up until now, is kind a big deal... Either the neighbouring nation could interpret it as hostile action preparing for an invasion where there are no defenses. Or their own nation interprets it as creating a new vector of danger to their nation that they have to invest resources into securing. Or outright assumes you are a traitor that intends to invade the other nation to invade through your territory like that.