For a medium medieval castle with towers, consider the following design ideas:

1. Castle Layout: A rectangular base with a central courtyard. Include two prominent towers on opposite corners for a balanced look.

2. Materials: Use stone bricks for the main structure, cobblestone for the towers, and wooden planks for the interior floors. Incorporate dark oak wood for doors and accents.

3. Towers: Design the towers to be slightly taller than the main walls, with crenellated tops for a classic medieval appearance. Add ladders or spiral staircases inside for access to the viewing platforms.

4. Walls: Construct thick stone walls with arrow slits for defense. You can add battlements along the top for authenticity.

5. Entrance: Create a grand wooden double door as the main entrance, flanked by torches or lanterns for illumination.

6. Features: Include a drawbridge over a small moat, a garden area with crops and flowers, and a blacksmith workshop adjacent to the castle.

7. Interior: Design the interior with a great hall featuring a long dining table, a throne for the ruler, and a fireplace. Add living quarters for the castle inhabitants.

8. Decorations: Use banners, paintings, and armor stands to decorate the walls and give the castle a lived-in feel.

This design combines functionality with aesthetic appeal, making it a perfect medium-sized medieval castle for your Minecraft world.