Mihrab - the prayer niche is the most important part of a mosque. But it unlike the "alter" in church and "Vedi" in the temple it is not a prerequisite structure for ceremonies or prayers.
Mihrabs are constructed to indicate the direction of Kaba. In Islam, congregation prayer should be led by an imam - the leader.
Anyone amongst the congregation can come forward and lead the prayer provided he should be able to recite at least some verses from the Quran and should also himself know how to pray. While in a congregation, it is mandated that imam should lead the prayer while stationed ahead of everyone else, and no one else stands to flank him from either side. The rows are formed behind him.
Mihrab serves in this functionality. It takes a form of a giant niche, deep enough to accommodate a prostrating man.
Since Islam prohibited imagery of any living beings, mosques are always decorated with carving or painting in geometrical designs, flowers, vines and calligraphy.
Since by implication, the mihrab is the most important part of the mosque, they are often decorated distinctly.
Mihrabs are not only constructed in mosques, but they can also be found within the mausoleum chambers of tombs. Latter, however, is shallower then what we see in a mosque.
The mihrab within Iltutamish's tomb is a lofty one and is constructed in marble. Carved by Indian artisans with supervision from master engravers with an understanding of the different variations of scripts for Arabic.
The stone was intricately carved with flowers, bel - boote ( vine and buds) and geometrical patterns. Since elements such as Kalash's and Lotus were already wildly used in religious and secular construction in Indian continent and they did not depict imagery of living being, they were accommodated here too.
Interestingly, the quranic verses are carved here in two different variations of Arabic script. One is rigid Naksh, and the other is Kufic.
Kufic interestingly can be considered the most interesting scripts. A proficient calligrapher can give letters shape of several geometrical objects.