Name the two cities which had fire altars.

Hint: Both these cities are a part of the Indus Valley Civilisation and were discovered by S.R. Rao and A. Ghosh in 1959s. Fire altars were narrow, raised outdoor enclosures, like a table. They were made of stone or bricks and were used for ritualistic purposes when a big fire had to be arranged.

Complete answer: The two cities which had fire altars are Lothal and Kalibangan.
Lothal is situated in the Sarangwal district of Ahmedabad in Gujarat. It was discovered in 1957 by S.R. Ghosh, an Indian archaeologist.
Very true to a typical Indus valley city, the site has a citadel, a lower town and a dockyard. Lothal’s significance can be attributed to it being a centre of bead making in the ancient civilization.
Among other notable discoveries at Lothal are the twin burials, painted pottery, gold beads and one fire altar.
Kalibangan is situated in Hanumangarh near Ganganagar, Rajasthan. It was discovered by A. Ghosh. The most notable discoveries from Kalibangan have been the Linga and Yoni, the symbols of Hindu god and goddess, Shiva and Shakti. In terms of agriculture a wooden furrow and tilled land, fortunately very well preserved have been found during excavation,
Five fire altars have been found at Kalibangan.

Note: These fire altars found at the two ancient sites have been linked to ritualistic purposes, the exact use is unclear. S.R. Rao links them to the worship of fire, Lord Agni. Fire altars have also been found inside the houses at Kalibangan, they were smaller in size than the outdoor ones but this definitely hints towards fire worship.