Both sites visited just recently (November 2017) with a rented car as that is the only possibility to reach San Lorenzo castle without participating in expensive organised tours.

From Colon one should folow the signs to Gatun Locks or Agua Clara (which is a new part of extended Panama Canal next to Gatun Locks). So far crossing Panama Canal is possible only through the Locks (althoug a new big bridge over the Canal is visibly going to be built). The passage is occasionaly closed due to ships going through the Canal but the crossing usually does not take longer than half an hour.

After the Locks there is an asphalted road towards Fuerte Sherman (there is a checkpoint where you should tell you are going to Fuerte San Lorenzo). A few km before the castle there is an official entrance to San Lorenzo Protected Area (5 USD per person for foreigners).

The fort itself alhough it was detroyed by Morgan and other pirates looks much better preserved than Portobelo fortifications. Full layout is still visible as well as the moat cut in the rocks. There are excellent views from castle walls (Rio Chagres, Atlantic coast). And because of relative difficulty to reach the site it is substantialy less crowded than Portobelo.