Shah Jahan, the Mughal emperor, built many magnificent buildings and cities in his reign. However, the most splendid capital built by him was Shajahanabad, also known as Old Delhi.

Architecture and Design of Shajahanabad:
- Shajahanabad was designed by the Mughal architect, Ustad Ahmad Lahauri.
- The city was built in a grid-like pattern with wide, straight streets and lanes.
- The main street of the city, Chandni Chowk, was 1.5 miles long and 40 yards wide.
- The city was surrounded by a high wall with 14 gates.
- The Red Fort was the centerpiece of the city and was built using red sandstone.

Monuments and Buildings in Shajahanabad:
- Jama Masjid: One of the largest mosques in India, built by Shah Jahan in 1656.
- Red Fort: The residence of the Mughal emperors, built by Shah Jahan in 1639.
- Chandni Chowk: The main market of the city, lined with shops and vendors.
- Fatehpuri Masjid: A mosque built by Shah Jahan's wife, Fatehpuri Begum.
- Shri Digambar Jain Lal Mandir: A Jain temple built in the 17th century.

Importance of Shajahanabad:
- Shajahanabad was the capital of the Mughal Empire for over 200 years.
- It was an important center of trade and commerce.
- The city was a symbol of the power and wealth of the Mughal Empire.
- The monuments and buildings in Shajahanabad are a testament to the rich cultural heritage of India.

In conclusion, Shajahanabad was the most splendid capital built by Shah Jahan. The architecture, design, and monuments of the city are a reflection of the grandeur and opulence of the Mughal Empire. Today, Shajahanabad is an important historical and cultural site in India and attracts tourists from all over the world.