This is one of the most crucial buildings in the early game. Allowing you to get border pops for just the price of 30 hammers or a forest without having to wait for expensive building like libraries or to bother with a religion gambit. Granaries have probably been discussed to death already as probably the most powerful building in the game but there are very little about monuments. With charismatic you also get +1 happiness in all cities which is huge in the early game for such a cheap price.