Turmezistan is a nation on Earth, it is in Central Asia. The country is also regarded as a failed state by the UN which has had to have a peace keeping mission in the country to prevent conflict from erupting.
During the Zygon rebellion in 2015, the rebel Zygons from the splinter group Truth or Consequences had a training camp hidden in a town in Turmezistan. The Zygons had replaced everyone in the town, and imprisoned Petronella Osgood there after they captured her.
UNIT dispatched the Twelfth Doctor to aid the UNIT task force already in Turmezistan- the task force was fighting the Zygons however its efforts had been met with mixed success- with the rescue of Osgood. However, when a squad of UNIT soldiers along with the colonel- who was in charge of the whole operation- along with the Doctor attacked the town, the rebel Zygons were able to lure a squad of UNIT soldiers into an ambush by impersonating their family members, killing all of the soldiers. The Doctor found Osgood hidden under the town church and fled with her as UNIT bombed the town, thereby destroying the Zygon base. (TV: The Zygon Invasion)
The United Nations had a base in Turmezistan, in order to ensure that the country remained free of conflict. In 2017, when a pyramid mysteriously appeared in a potential war zone, the Doctor was summoned. Along with Bill and Nardole, the Doctor investigated the pyramid. (TV: The Pyramid at the End of the World)