The bridge is just over half-way from the city area to Belem, so it was good to pass.
The Belem Tower is a UNESCO World Heritage Site (along with the nearby Jeronimos Monastery) because of the significant role it played in the Portuguese maritime discoveries of the era of the Age of Discoveries from the early 15th century to the 17th century; this would seem a nicer (and somewhat shorter name than the potentially more accurate and appropriate but less politically correct "Age of Taking Whatever We Want Whenever We Want From Wherever We Want". The tower was commissioned by King John II to be part of a defense system at the mouth of the River Tagus river and a ceremonial gateway to Lisbon.
Built in the early 16th century, the tower covers 5 levels, including the underground storage area - used to store materials, equipment, and prisoners. The rest of the tower provides significant areas of fortified defensive positions.
Moving up through the levels was reasonably easy due to being the low season, but I can imagine it would be terrible in the high season; the tower itself is limited to 120 people at any one time (with turnstiles controlling entry - one out, one in), and a light system, like on road-ways, controlling the traffic up and down the narrow circular staircase in one direction at a time.
The construction of the monastery and church began on 6 January 1501 (and were completed 100 years later). King Manuel originally funded the project with money obtained from the Vintena da Pimenta, a 5% tax on commerce from Africa and the Orient, equivalent to 70 kilograms of gold per year, with the exception of pepper, cinnamon and cloves (which went directly to the Crown. With the influx of riches, the architects were not limited to small plans, and by the looks of it, they took that quite literally as the result is anything but small.
The only areas open to the (paying) public are the Church of Santa Maria and the cloisters, but even these are beautiful and impressive. The church contains the tomb of Vasco da Gama (of finding the route to India, and therefore opening that trade route fame); on the upper level choir is a statue of Christ on the cross dating to 1550.