The “Torre delle Quattro Porte” (4 gates Tower) - as this tower is popularly called – even if it would be more correct to call it “Porta di Riale” (probably deriving from the late-Latin word “rivalis”, coming from the Latin word “rivus”, that is river) is one of the most unusual examples of medieval tower building in Vicopisano. It is 15 m. high and 6.4 x 6.4 m wide at the base; its main feature consists in being a watchtower and an entrance gate to the Castle (the only one still existing) at the same time: on each of the four sides of the tower, a pointed arch gate opens up, which in ancient times gave access to - and allowed to walk along - the inside of the Castle walls.
 
On the north gate, there are two big stone hinges - still well preserved - that in Middle Age allowed the opening of the massive castle doors. Due to the fact that it was an entrance gate and thus one of the main targets for attacks, it shows 4 distinctive building phases, also highlighted by the use of different materials: “verrucano” stone and terracotta bricks. These building phases date from the end of the 12th century to the 15th century, after the conquest by Florence. In 1939 the tower embattlement was restored.
A scarcely known detail is that the tower risked to be destroyed towards the end of 1700, while the north side walls were being demolished. In fact, it was going to be sold by the Municipality to a private, who had shown the intention to destroy it. Fortunately, it was decided to lease it out, binding the lease to the obligation of preserving the four doors.