Standing on a small island just off the shore in the Tagos River, Belem Tower (Torre de Belém, in Portuguese) is a remnant from a bygone age when Portugal's ships ruled the waves. It is one of Portugal's iconic landmarks, and you'll see it represented everywhere from t-shirts in the souvenir shops to the ornate stained glass windows at Pena Palace in Sintra.
The tower has surprising decorative flourish--it was, after all, the gateway to the vast Portuguese empire. But its thick limestone bricks are impressively solid, in keeping with its primary purpose. It was part of a defensive net to protect Lisbon Harbor, the hub of Portugal's trading and naval empire. The tower, above all, served as a fort, lookout, and platform for dozens of heavy-duty cannons pointed out over the Tagus.
It was built during Portugal's golden age of discovery, from 1514 to 1520. It was a period when Portuguese mariners like Vasco da Gama were mapping the coasts of Africa and India and establishing a decades-long Portuguese monopoly of those trade routes, often with brutal methods.
Photos of Belem Tower
I have more photos of Belem Tower here.
Paired with the Jerónimos Monastery, Belem Tower was added to the list of UNESCO World Heritage Sites in 1983. It's very easy to see from the shore, and when it's open during the day you can walk the short wooden bridge for a tour.
Travel Advice for Portugal
You can find the latest U.S. Department of State travel advisories and information for Portugal (such as entry visa requirements and vaccination requirements) here.
Health & Vaccinations
The CDC makes country-specific recommendations for vaccinations and health for travelers. You can find their latest information for Portugal here.