The four storeyed main gateway of the Taj Mahal is 100 feet high and is built in red sandstone. The Arabic calligraphy from the Holy Quran and motifs of entwined flowers, leaves and vines spiraling down its niches. These motifs have been made by semi-precious stones inlaid in the white marble. The makers employed an optical trick so that as you move closer to the gate, the Taj keeps getting smaller. It seems to grow bigger as you walk away.
Source: https://www.scoopwhoop.com/inothernews/lesser-known-taj-facts/#.39cu3uxre, http://www.tajmahal.gov.in/entry.html