This answer assumes that you are actually living on a desert far off nearby waters
You basically lose your chances of waging war towards the kingdoms that are near the coastline, because of Water
Technological advancements would be a pain, specially since if your age would start on somewhere from primitive to medieval age, water helped craft metals which is essential for your civilization to advance technologically. And water is essential to every day life so....
So, what could you do?
Rather than invading them, WORK with them, that way you'll have access to water, which would then provide you to somehow enhance your technology and build your empire or your country.
BUT you still want to CONTROL them right?
Then control them in a different perspective.
All of their lives they live in a paradise different than yours, culture, arts and even science. You must "showcase" your culture, arts and science as beautiful as you could, as true as you could as the best as you could, and if the people from the coast accepts those, then that could be one way to control the other kingdoms.
Here are some examples to elaborate my point.
Religion: Rome, Italy - Vatican City. The place where the pope lives. The Pope "controls" (in a way) how catholics worship. One example is Pope John Paul II introduced the Mysteries of Light.
Culture: Japan's culture is widely envied, accepted and loved worldwide (specially their anime), yes they lost the war, but they won the hearts of many people, including the nation that dropped the bombs wink.
Arts: African American Rap music is regarded as one of the most "influential" arts as it somehow "dictates" the way someone wishes to live, from being a hustler, a baller, a gangster. Even the way they speak is influenced by this.
Science: Albert Einstein is a German theoretical physicist. IF Hitler just produced a million or more of this guy (Not going to "clone" him, but let other nations LEARN from him) Germany could have influenced more countries in the field of science and maybe, dictate the flow of technology advancements.
TL:DR War is not the only option to control other countries, there are plenty of other way to influence them, essentially controlling them without them knowing it.