The people of the Lost Valley have a unique technology style. While their architecture and clothing is reminiscent of Medieval European styles, they utilize much technology (especially weapons) that we would associate with futuristic sci-fi. Examples include:

Projectile firearms (small arms)
Holographic projections for communications and surveillance
Robotics
Electric furnaces
Ballistic projectiles and explosives (siege weapons, i.e. cannons)
Energy weapons (though these are still experimental and not in general use)

Both sides are constantly making improvements and advancements on their weapon technology in an attempt to out-gun the other side. Because the entire society of the Valley revolves around the War, everything is built for efficiency and quick construction and repair. Simpler weapons, such as swords, crossbows and spears, are still in general use, as they’re quicker and easier to produce en masse than firearms.

No, no time traveling. At first I was thinking they had developed the more advanced weapons and tech during the war, but an idea just occurred to me that perhaps they started off with the more advanced technology, but the ravages of the war have cause them to rely on more primitive, easier to produce, weaponry.