Temerian Quarter is a district of Loc Muinne. Geralt only gets full access to it if he follows Roche's path. It is also there that he has his final meeting with Letho.
During the summit of mages and monarchs, the camp of the Temerian delegation led by John Natalis extended over an entire district of Loc Muinne. Plenty of diplomats milled around the camp, but soldiers were present in far greater numbers.
If you have chosen Iorveth's path, visiting the Temerian Quarter will cause an infinite amount of soldiers to spawn and to fight. It is recommended to make a save prior to entering the district, as you will be trapped in there once you've entered.