Approach to the answer: 

Understanding and structuring the answer: Provide a comprehensive response to the question about the important characteristics of Indo-Islamic architecture during the Sultanate period and the new elements introduced under the Mughals. 

Introduction: 

Type 1: Briefly introduce the topic of Indo-Islamic architecture, emphasizing its development during the Sultanate period. 

Type 2 : Explain how Indo-Islamic architecture was unique due to contributions of Sultanate and Mughal phases.

Body: 

Heading 1: Indo-Islamic Architecture during the Sultanate Period

Heading 2: New Elements Introduced under the Mughals

Conclusion:

Type 1: Summarize the key characteristics of Indo-Islamic architecture during the Sultanate period, which laid the foundation for the architectural innovations introduced under the Mughals.

Type 2: Emphasize how Mughal architecture combined elements from various cultures to create some of India's most iconic structures, reflecting a fusion of artistic and architectural excellence.

Indo-Islamic Architecture during the Sultanate Period:

• Use of Red Sandstone: Sultanate architecture often employed red sandstone for construction, seen in iconic structures like the Qutub Minar in Delhi.

• Arches: The use of pointed and horseshoe arches, a hallmark of Islamic architecture, became prominent in buildings like the Alai Darwaza.

• Minarets: Minarets with intricate detailing were constructed to call the faithful to prayer, and some, like the minarets at the Qutub Complex, showcased a fusion of Islamic and Indian styles.

• Calligraphy and Geometric Patterns: Intricate calligraphy, often in Arabic script, and geometric patterns were used for decoration, as seen in the Alai Darwaza and the Tomb of Iltutmish. 

New Elements Introduced under the Mughals:

• Use of White Marble: The Mughals introduced the extensive use of white marble in architecture, exemplified by the Taj Mahal and the Moti Masjid in the Red Fort.

• Pietra Dura Inlay: The technique of pietra dura, where semi-precious stones were inlaid into marble to create intricate designs and floral motifs, became prominent during the Mughal era.

• Large Domes: Mughal architecture featured large bulbous domes with intricate floral designs and finials, like the ones found in the Badshahi Mosque and the Humayun's Tomb.

• Charbagh Gardens: Mughal gardens, designed in the charbagh (four-fold) style, became an integral part of Mughal architectural complexes, adding a sense of symmetry and aesthetics.

• Double-Dome Structures: The Mughals introduced double-dome structures, where an inner dome provided structural stability while an outer ornamental dome added grandeur.