For your medieval castle design using cobbled deepslate, consider the following ideas:

1. Castle Layout: Create a large rectangular base with a central courtyard. Surround the courtyard with high stone walls made of cobbled deepslate.

2. Towers: Incorporate four corner towers, each rising to a height of at least 10 blocks. Use cobbled deepslate for the main structure and add crenellations at the top for a classic medieval look.

3. Moat: Dig a moat around the castle, at least 3 blocks wide, and fill it with water. Consider adding a drawbridge at the entrance for added authenticity.

4. Gardens: Design a series of gardens within the courtyard. Use grass blocks, flower beds, and hedges made from leaves to create a lush environment. You can also include a small fountain as a centerpiece.

5. Entrance: Build a grand entrance with an arched doorway, flanked by two smaller towers. Use cobbled deepslate for the arch and add wooden doors for contrast.

6. Interior Features: Inside the castle, create a great hall with high ceilings, large windows, and banners. Include a throne room, dining area, and living quarters for added detail.

7. Lighting: Use torches or lanterns to illuminate the castle, placing them strategically along the walls and in the gardens.

This design will provide a stunning medieval castle that showcases your creativity while utilizing cobbled deepslate effectively.