One of his projects was a huge mosque in Samarra along the Tigris River. His desire for ostentatious displays of wealth and devotion made it the largest mosque in the world at the time, and it was completed in only three years.
The minaret (shown here) is 171 feet high and 108 feet wide, the top reached by a spiral ramp. It is all that remains of the original structure. The main building was constructed from baked brick octagon piers with four columns in the corners of imported marble. It had 17 aisles, and the walls were covered with dark blue tiles.
A total of 16 doors allowed the faithful inside, where light was provided by 28 windows, 24 of were oriented by the qibla, the direction to face when praying. A fountain in the center was believed to be carved from a single stone and delivered by elephants. That had been commissioned by al-Mutawakkil's predecessor, his brother al-Wathiq. The baked brick ceiling was 35 feet high, supported by 464 pillars.
In 1278, the mosque (but not the minaret or outer wall) was destroyed when Iraq was invaded by Hulagu Khan's Mongols. A restoration process started in 1956.
The construction of the Great Mosque was part of a plan to make Samarra the center of an expansive urban area, so there were other projects in the area. Part of this expanded area was named al-Mutawakkiliyya for himself. Another new city he founded needed water, and he commanded two courtiers, the brothers Muhammad and Ahmad ibn Musa, to make it happen. They ignored local engineers and turned instead to al-Farghani, an astronomer. He was very smart, and a good mathematician with a good reputation, but he made a tiny mistake with great consequences. That's a story for tomorrow.