Portchester Castle is in Hampshire and is on Portsmouth Harbor. Its King Arthur connection is somewhat tenuous and rests on this Welsh poem from the Black Book of Carmarthen
In Llongborth I saw Arthur's
Brave men who cut with steel,
The Emperor, ruler in toil of battle.
This poem is a Welsh elegy about a prince named Geraint, who was the ruler of a region called Dumnonnia in the late 5th century. This included much of Southern England as far as Cornwall.
Llongborth translates to warship port, which could place Llongborth in Portsmouth Harbor. The stronghold then becomes Portchester Castle, which is a fortress immediately on the harbor and originally built by the Romans. Eventually, the Saxons conquered the fort and used it for themselves.
Somewhat convuluted, and unlikley that Portchester Castle was the only "warship port" in that kingdom at that time.
Southern England Arthurian Sites