Why would the ruler leave?

It would be natural for the leaders to want to find out about lands close to them. It would have been long known that there was a mainland, and possibly visible, as Calais is from Dover (on a good day!). However, the mainland must be difficult to pass. I think that if it were mountainous then there would be a reason why other nations had not got in contact. A desert would be too hard to set up a trade route through.

A number of surveys of the mainland were made to test the mainland then try and find passages over time, possibly over 100 years or longer, could have led to discovery of routes and eventual contact with others. This would mean finding areas in the mountains that were suitable for colonisation, due to requirements of traveling (running out of rations).

Existing small villages could be an alternative, although they would also need reasons for not being interconnecting, unless there were a combination of both. In fact, there would likely be a thinning scattering of villages in the mountains from the direction of civilisation on the other side. It is likely that they will have strange beliefs due to the remoteness.

If a previous expeditions to map out possible passages on the mainland to other civilisations succeeded then the ruler may need to attend for diplomatic reasons. In fact this is likely in early stages of contact to determine whether hostile or likely trade allies. The find would have to be a major town or city, perhaps a member of an empire.

This is not far off how we think about space exploration today and is often used in science-fiction. Typically with colonies being used as spring boards to more and more remote places.