the Kremlin to be torn down. In the following centuries Moscow expanded rapidly outside the Kremlin walls and as Russia's borders became more and more secure their defensive duty has all but passed. The cannons which were installed in the walls were removed after the turn of the 17th century, as was the second, smaller wall which repeated the perimeter on the outside. During the reign of Czar Alexei Romanov, the towers were built up with decorative spires and the walls were restored. However their historical mightiness was dampened as the material became brick not stone. Successive restorations of varying scale took place during the reigns of Empress Elizabeth and Alexander the First as well as the later Soviet and Russian times, preserving their original character and style. Specifications With an outer perimeter of , the Kremlin appears as a loose triangle, deviating from the geometric ideal on the southern side where instead of a straight line, it repeats the contours on the original hill on which the Kremlin rests. Because of this the vertical profile is by no means uniform, and the height at some places ranges from no more than quadrupling to elsewhere. The thickness of the walls also varies from . The top of the walls, along their entire length, have outwardly-invisible battle platforms which also range from in width (in proportion to the thickness). A total of 1,045 double-horned notched "teeth" crown the top of the walls, with a height ranging from and thickness from . Some of the interior corridors inside the walls have rooms with no exterior illumination () where particularly dangerous criminals were contained. To date twenty towers survived, highlighting the walls. Built at a different time, the oldest one, Tainitskaya dates to 1485 whilst the newest one-Tsarskaya to 1680. Three of the towers, located in the corners of the castle have unique circular profiles. From the ground level it is only possible to enter six of the towers, the rest only from the walls. Four gate towers exist, all crowned with ruby stars, they are Spasskaya, Borovitskaya, Troitskaya and Nikolskaya. Although up to the 1930 it was also possible to enter the Kremlin via the gates of Tainitskaya tower, however these were covered up yet leaving their portal clearly visible. The main gates in the Spasskaya tower are normally (with the exception of official and religious ceremonies) closed to the public. The