If you go to Belem from Lisbon, whether you drive, take a cab, or the tram, you will pass the old rope factory on your way. The road to Belem runs along the River Tagus, with renovated warehouses on your left (many now converted to restaurants and nightclubs). On your right, shortly before reaching Belem, you will pass a LONG (block is long!) bright yellow building on your right. This is the old rope factory. Since a splice is a weak point in a rope, and since rope was so critical to the economy of Portugal during the voyages of discovery, they needed the ability to manufacture very long ropes without splices. Hence, this building, which may very well be the longest building anywhere!
There are apparently no tours of the rope factory, and in fact I do not know what the building is used for today, but it is well worth watching for it on your way to Belem, because it is quite a marvel.