Complex sentence: This was established soon after 43 CE on land then a mile from the sea at the north end of the Wantsum channel, which once separated Thanet from mainland Kent, and it was probably there to defend the Roman fleet anchored in the channel.
Split into simpler sentences: This was established soon after 43 CE, on land which was at that time a mile from the sea, at the north end of the Wantsum channel. The Wantsum once separated Thanet from mainland Kent, and the fort was probably there to defend the Roman fleet anchored in the channel.