I spent a few days in Old San Juan before Christmas last year; the sentry boxes (las garitas) are all along the walls of Fort San Cristóbal and Fort San Felipe del Morro. It's easy to see why they became a symbol of Puerto Rico. (Since that was the Territory Quarter, the America the Beautiful Quarter is going to be El Yunque National Forest.)

As for the new series, I find it amusing that Puerto Rico is #11 while Massachusetts is #46 (since the Lowell NHP wasn't created until 1978).