Well, keep it simple and sturdy. Thick stone walls, high enough that even a large beast can’t get past them easily. Add a narrow, steep approach that makes it hard to throw a rock at you. Put watchtowers on the corners so you can spot trouble early, and leave a small gate that can be closed quickly. If you’ve got the skill, line the walls with some runes or a ward that slows or deters magic users. A shallow moat or a ditch around the base can keep the bigger creatures from getting close, and a good supply of food and water will keep your guards and travelers fed when the night comes. That’s about it.