There is a promenade which runs along the Tagus and you can walk from the Monument of Discoveries to Belem Tower. The Tower is a UNESCO World Heritage Site and served as the home of the Captain of the Navy, a prison for traitors and a fort from where to defend Lisbon as it is positioned at the mouth of the river Tagus.
You can visit the Tower and it is definitely worth seeing. Its relatively small stature means it doesn’t take too long to tour it. Try to behave or you might just get locked into its formidable dungeons!!!
Take the train from Cais do Sodré to Belem and cross the road to the promenade or take tram 15 E from Praça do Comércio heading towards Belem to get to the Monastery, Tower and Monument.Map Location: Belem Tower