The construction of the moscow kremlin as a red-brick stone fortress began in the late 15th century. at that time ivan iii ruled over the united russian state, and he invited italian architects to supervise the reconstruction of the kremlin. the newly-built stronghold was a massive brick wall with 20 towers. it surrounded the most important buildings, the construction of which was continued later. now the kremlin is one of the most ancient parts of the city. each corner has one tower. the tallest tower of the whole kremlin complex is the the troitskaya tower. one of the most popular towers is the spasskaya tower. its senior architect was the famous pietro solari. the main kremlin clock is situated here. the kremlin is also the the seat of the russian government. the grand kremlin palace is the principal building, and the through-passage to it is located in the nikolskaya tower. the complex serves as the residence of the president of the russian federation. the ivan the great bell tower is renowned for once being the tallest building in old moscow and the tallest belfry in russia. the tsar bell was installed nearby. it is the world’s largest bell and it weighs about 200 tons. the famous tsar cannon was constructed in 1586.