Nueva Castile was somehow almost completely isolated from the Inner Sphere from late 24th century until 2830 when the BattleMech equipped "Umayyads" invaded their worlds. The Umayyads were so technologically advanced that (despite their small numbers) they conquered most of the Nueva Castile worlds until their infighting stopped their advance before they conquered the last remaining Castile world of Asturias in 2855.
In 2857 the Nueva Castile forces managed to capture a BattleMech and by 2862 the Castilians had not only reverse engineered the technology to a point where they could manufacture primitive ICE powered BattleMechs but also produced enough of those Mechs on their single remaining planet to turn the tide of war. Over the next 200 years they would conquer back most of the lost worlds from the Umayyad Caliphate.
I couldn't find much on these reverse engineered Mechs on Sarna or by googling, just that they were primitive and Internal Combustion Engine powered. Nueva Castile worlds were established about 50 years after Myomer and industrial mechs were invented so most likely they had that technology available to them. BattleMechs however were a completely new concept and Nueva Castilians had generally lived peacefully for the last 400 years. Those Castile made Mechs were still good enough to turn the tide of war against an enemy that was either a Clan Wolverine scouting force, or some Inner Sphere group fleeing from the 2nd Succession War.
Has there ever been any mentions (or perhaps even TROs) of what these early Castilian Mechs exactly were? I find this whole conflict one of the most interesting in BT Universe (narratively, technologically and as a potential campaign/battle setting), although it's best not to think too much about how an agrarian Deep Periphery backwater faction reduced to a single planet over the last decade somehow reverse engineered and manufactured their own BattleMechs in five years from a single example while the Inner Sphere powers couldn't really do it in 200 years...
