I'm very glad to hear you enjoy the game, ha ha!
About your question, when you go to the world map, search for a gate on the floor in the north. When you interact with that one, you should be able to get into the sewers! For further help, you can also visit the wiki and see if you find anything you need there https://thebestmaster.fandom.com/wiki/TheBestMaster_Wiki or you can also join our discord, where we have a lot of really helpful veterans at this game that love to give advice! https://discord.gg/HxTXwK