Later, quarries of soft limestone were found in abundance, and from these, blocks were easily cut which, after exposure to the atmosphere, formed a hard, durable building material. The coral rock of which both Morro and Cabañas were built was taken from old quarries scattered along the north shore from Morro eastward. From these quarries came also the stone that built the spacious San Francisco Convent, occupied today by the Central post office. As in all Spanish towns, in the New World at least, a plaza or open square formed the center from which the principal streets radiated. On the eastern side of the plaza of Havana, in front of La Fuerza, was erected in after years El Templete, in honor of the first mass held by the inhabitants of Havana, which took place under a giant ceiba growing close to the shore of the harbor, in 1519. Nearly all of the permanent structures in Havana, up to the middle of the 17th century, were located on or near the water front, some distance in from La Punta. Many of these, including La Fuerza, the San Francisco convent, the old cathedral and La Maestranza, were built of coral limestone cemented with a mixture the formula for which is said to have been lost, but which in these buildings has endured the wear of centuries. Excellent clay for making tile and brick was later found not far south of the City, so that the more pretentious buildings were covered with roofs of the criolla tiles that are still common throughout all Latin America. Before the middle of the 15th century, the clearing in which Havana was located was extended out as far as the street now known as Monserrate, running from the Gulf front across to the southwestern extension of the bay. In 1663 a splendid wall was begun along this line and completed with the help of slaves in 1740. It ran almost north and south, inclosing the city on the west, and protected it from all attacks coming from the land side. This wall was twenty feet in height and twelve feet thick at the base, surmounted at frequent intervals by quaint round-topped turrets. It had its angles, bastions and points of vantage for defensive purposes, the work, according to experts, representing a very high degree of engineering ability on the part of those who planned it. With the exception of one angle and its turret, which stands in front of the new Presidential Palace, the old walls were removed in 1902, thus depriving Havana of perhaps the most picturesque feature of the ancient city. Just in front of this wall on the west, a wide clearing was made to prevent surprise attacks from the forests beyond. With the felling of the trees, grass soon grew along its entire length, hence the name Prado, which means meadow, became permanently attached to it, and so the green lawn in front of the old walls of the 17th century was transformed two hundred years later into Havana’s most aristocratic avenue.