Short Version: If everyone in my army is going to be armed the same way, what is the most effective weapons system that isn't a horse archer? It is understood that this is still not as good as combined arms armies with more specialization, but for story reasons please just go with it.
Long Version: To my knowledge, and please correct me if I am wrong, the Mongol Golden Horde was able to conquer a fair bit of Asia with an army made up almost entirely of horse archers (as well as some siege weaponry at times). Their skill, as well as their ability to attack from range and outflank enemies with great speed, made them arguably the greatest fighting unit of the Ancient world. Again to my knowledge, they didn't have any major weaknesses because anyone who was armored enough to avoid dying by arrow fire had no chance of catching them, and if chased they could retreat into the Steppe until your supplies ran out. This has me wondering what the greatest weapons system in an ancient world setting could be, outside of horse archers.
In my story there is a nation that arms 95% of their military in the same way for reasons. While this is certainly not ideal, it is their only option. I have considered the entire army just being excellent archers, but I'm not sure that's the best solution. I have also considered Hoplites, but again not sure whether that is the best course of action. The solution can't be horse archers, if only because I want it to be slightly more exotic than the familiar raider stereotype. The uses for this military will be primarily defensive, fighting both in open pitched battle and defending walled cities, with occasional requirements for offensive engagements. They will be facing multiple combined arms armies similar to troops from the Ancient Era. Flexibility and being at least average at most things is therefore a must. With again the understanding that this army on a practical level is at a massive disadvantage, how should I arm them to give them the most realistic chance at defending their nation with only one troop type for 95% of their army?
Edit: Nzaman pointed out that the environment is crucial here. Therefore, this is an area about the size of Ireland, with mountains surrounding it. Other than the mountains and foothills, there is a very large almost impassable swamp, and some gentle hills giving way to plains.
Bonus points if the answer includes what the 5% "Special Forces" units should be armed with to act in support.
Addendum: Possibly unnecessary info, but the world in question has magic to some degree. I would prefer the military strategy mostly avoid using this, but if a little magic can aid in an interesting answer then go for it.