Pixie R.
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First State NHP's New Castle site was very interesting, and I learned a lot while there. Definitely check their hours before going, because they're only open Thursday through Saturday, 9 am to 4:30 pm, and the tours are worth going on, in my opinion. There's a whole historic town you can walk through (maps are in the Visitor Center, AKA the Arsenal), and my mom and I didn't even get through the whole thing in one day. I would recommend spreading your visit across multiple days to get the most out of it.
Our tour guide for the Courthouse Museum, Ranger Lexie, was very friendly and made the quick, 15-minute tour very engaging. She also took the initiative to do a quick internet search for questions she didn't know the answers to. I was very impressed by the many exhibits in the Courthouse Museum; the signs were well-made, and some were interactive. I learned a lot about America's history, even the problematic parts. It is clear that the people who work here care deeply about educating the public on history for free.
There was also a woman who gave a very interesting and thorough 45-minute tour at the Courthouse itself, but I forgot to get her name! I learned a lot from her about the people for whom the University of Delaware's residence halls were named, e.g. Louis Redding and Caesar Rodney.
If you have your National Parks Passport, you can get stamps from three different sites: the Visitor Center (AKA the Arsenal), the Courthouse Museum, and the Courthouse building!