mjhjmtu

Pangea in this setting never broke apart into different landmasses. Instead it stayed together as one giant super continent. As a result, most human civilizations developed along the coastline, where the most fertile land is. Because human society is in relative proximity to each other, cultures and religions became more homogeneous and globalization occurred far sooner than in our world. travel and communication became easier, with sea travel being the most common as one could theoretically get to any civilization by boat.

Inwards of the super continent are vast regions of desert. This is where marginal and disenfranchised people have been known to reside (exiles, defeated kingdoms, criminals, etc) Over time, this developed into a civilization of its own. It has dreams of making war with its neighbors and establishing a world wide empire, but is at a significant disadvantage due to its lack of resources.

How could an empire developing this deep into Pangea gain control of its neighbors?

The Mongols are a Good Example

What really made the mongols so effective at expansion was how they were willing to incorporate the people they conquered into their plans to conquer their next target. Thanks to a campaign of brilliant (and brutal) psychological domination the mongols often utilized captured specialists to perform tasks for them that they themselves could not do. This is how despite being a rather simplistic and backwards tribe of nomads from the barren steppes they were able to construct huge siege engines and vast navies. That is to say, they were able to force their conquered subjects to build such things for them. All your inland fledgling empire needs to do is conquer one of the coastal empires and then through psychological and physical domination force them to build and crew the ships for them. Everyone likes to think that they would be a hero and resist the invaders, but the reality is that if you are a shipwright and are faced with the choice between a very lucrative and generous contract with the new rulers, or having everyone in your family and anyone who even vaguely knows you be raped, tortured and executed you tend to rather happily accept the former option. Its not very nice, but its how some smelly barbarians on little horses were able to conquer damned near everyone they met.

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 they 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.