You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Genghis_Khan

Context:
The Mongols attacked Samarkand using captured enemies as body shields. After several days only a few remaining soldiers, loyal supporters of the Shah, held out in the citadel. After the fortress fell, Genghis supposedly reneged on his surrender terms and executed every soldier that had taken arms against him at Samarkand. The people of Samarkand were ordered to evacuate and assemble in a plain outside the city, where they were killed and pyramids of severed heads raised as a symbol of victory. Ata-Malik Juvayni, a high official in the service of the Mongol empire, wrote that in Termez, on the Oxus, "all the people, both men and women, were driven out onto the plain, and divided in accordance with their usual custom, then they were all slain".

Question:
What victory symbols did the Mongols build on the plains outside Samarkand?

pyramids of severed heads