A “dun” is a larger fortification, few and far between on the island of Ireland. We saw one on the Arran Islands, from the Iron Age, Dun Angus, Charles Fort, or Dún Chathail in Irish, is from historical ages.
A cannot tell from my slide show, but the walls are star shaped with many salients, giving more positions to defend the walls.
Steps to bridge across defensive ditch 
Bridge and gate to fort enclosure, closed this late in day 
Embrasures 
An opening in the fort walls which cannons can be fired. They were often flared outward to enlarge the field of fire. 
Close-up of gun port 
Crenellations 
Tightly joined masonry 
lichen 
life has its way 
seems to be a succulent 
a doorway here seems a bad idea, but what do I know? 
A pulley at first glance, look again and it is a lock 
beneath the walls 
Down to the Sallyport and the next post
References
“Charles Fort” – wikipedia
Michael Stephen Wills Photography 
Beautiful shots Michael!
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cheers, Francisco
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You’re very welcome Michael
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It’s interesting how many of the features are similar to one of our historic Texas forts:the Presidio in Goliad. Rice University has a sallyport that features in certain ceremonies like graduation, too — and there’s a Texas wildflower that looks for all the world like crenels and merlons as the bud emerges.
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Texas is big enough for all of that
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