I toured Fort Jackson several years ago, but I didn't realize there were fortifications on the east side of the river as well. It makes sense when I look at the river bend on Google Maps.
I gather you got to Fort St Phillip by boat, as I cannot see any roads or trails one could drive on in Google Maps. Looks like a pretty big area to explore.
Does it get submerged at times, it looks kind a muddy in your last image?