dragon bro, kathmandu was ruled by maithali brahmins for 1700 years before gorkhali thakuri ruled it.so after the kirat period, the maithali speaking lichhivis bought hindu religion,script, caste system,administration ,the costumes etc from south and ruled nepal valley.
after that came the mallas who were also maithal brahmins and made even stricter rules based on hindu dharmashastras .read jayasthiti malla and his nyayashastra where a jyapu or pode cant wear gold or silver,full sleeve dress or even walk on the middle of the road let alone touch the high castes.the language spoken in the royal palace was maithali and the rules took pride in the strictest form of hinduism bein followed in kathmandu. the buddhist newars and their religion were looked down upon more in malla period than in lichhivi times.
काठमाडौँ बनको तराईको बाहुन साशकहरुले हो, न कि ज्यापूहरुले.त्यो त राम तेली ले ताज महल बनायो भने जस्तो भो-most of the temples and monuments were build in malla period( i talk about the present form of them, not the original form for no one know how they looked originally in lichhivi period). os one just remembers the ruler who made some monument in history ,not the people. maybe the rulers being from south brought some artisans from south who fused the local art with theirs from south. but the credit still goes to the rulers.
well, regarding brahmins, there were brahmins before the shah conquest of valley,the maithali speaking jha brahmins were the ruling elite(ministers, priests)who are still there in valley. most of the elites of mall era were droven out including the kings family and relatives.
als there were some pahadiya brahmins as well in valley .there were also khas and magar generals since the time of pratap malla(according to his manuscript on rani pokhari).