This is a rather easy introduction if you did not kill the Baron’s men during the Nilfgaardian Connection quest. You’ll have a brief talk with the guard at the gate and it will be easy to convince them to let you pass.
If you killed the Baron’s men than the townspeople run and hide from you and you will have to find another way into the Baron’s castle. You can talk to an old man in the town who doesn’t seem to be afraid of Geralt or try to find the entrance yourself to a nearby cave yourself. Talking to the old man will cost you a small sum in order to get the info.
Overall the impact of you killing the Baron’s men does not go onto effect much else than how the villager NPC’s react to you during the first visit to the Baron's stronghold.
This quest will lead into Family Matters and the secondary quest Ciri's Room.

Embark on an inspiring mission of human connection beyond the UCA. Sam — with companions by his side — sets out on a new journey to save humanity from extinction. Join them as they traverse a world beset by otherworldly enemies, obstacles and a haunting question: should we have connected?