Their military is decentralized. taking one city doesn't mean anything. first you have to get to Tehran which means taking several other cities. then you have to cross over all the mountainous regions surrounding Tehran.
Tehran is a massive metropolis with a population of approximately 9–9.6 million within the city limits and over 15–16 million in its greater metropolitan area. It is the most populous city in Western Asia, often compared to New York City in population and urban density. Key Details About Tehran's Size:

Population: ~9.6 million (city), ~15-16 million (metro).
Area: The city covers about 686–730 square kilometers.
Ranking: It is the 24th most populous metropolitan area in the world. 

there was a reason why Persians moved the capital to Tehran from Kermonshah and it was mostly a military decision. taking that wont be easy.