Prontera is the capital of the Rune-Midgarts Kingdom. It is home to the Acolyte, Priest, Knight, and Crusader guilds. Prontera is the heart of most of the Ragnarok Online communities, as well.
At the North Part of the map (156, 357) is the access to the Prontera Castle, which leads as short-cut to the Valkyrie Guild Map. It also inhabits the Royal Operations Quest, RDC.
Prontera gives access to a lot custom NPCs, most notably to access the Helpdesk via the GM Assistant.
Further Sales and Event NPCs are placed here such as the Halloween item exchanger or the Black Friday Sale entrance NPC (mostly north of the center fountain).
Center Part NPCs (Left):
Center Part NPCs (Middle Part & the way south):