Basic culture building of the Ancient Era. Available from the start of the game.
The Monument increases the Culture of a city, speeding the growth of the city's territory and the civilization's acquisition of Social Policies. It's the first building available for every city, without having to research any technology.
Note that under normal circumstances (that is, without any social policies or civilization modifiers) a settled city starts without any Culture, which means its borders won't grow AT ALL. If border growth is needed for a city, the easiest way to get the culture requires is to build the Monument – hence the provision of one from Legalism, which saves a lot of time and production.
There are alternatives to building culture buildings like the Monument; the opening bonus of the two early social policy trees is similar in effect to those of a Monument. Cities may also gain some from Wonders, including the Palace as a national wonder in the Capital. However, the Monument is vital for continuing the chain of cultural buildings: it is required for the Amphitheater in the expansions (or the Temple in vanilla Civilization V).
With the help of alternative sources of culture and if enough early social policies have been obtained, it may be possible to sell the Monument midway through the game in order to temporarily save Gold. If one chooses to sell one's Monuments, they can be rebuilt later when players will have more funds to invest (such as after discovering Currency) and will soon have access to game-changing Social Policies (such as those of the Rationalism tree).
A monument is a structure built to commemorate an important person, event, deity, or concept. The more important monuments are usually constructed near the center of the city, by the ruler's palace, or near the city's main gates. Monuments come in all shapes and sizes, from the Great Sphinx of Giza, Egypt, to the Statue of Liberty in New York City, USA, to Nelson's Column in London, England. The best of them imbue the city's inhabitants with a great feeling of civic and national pride.