Port Tyras is a port west of the Tyras Camp. Players can't use the charter ships away before completing Regicide, but to arrive there by ship requires that you have chartered a ship coming from Port Tyras already OR if you have killed the Tyras Guard as part of the Regicide quest already. This is the fastest way to get to Tyras Camp other than walking through the dangerous elven forest. Some players may also find this helpful in getting to Lletya if they have lost their crystal teleport seed and do not want to go through the Underground pass or Arandar gate.

Port Tyras was likely erected as part of King Tyras' invasion of the forest of Isafdar, though it has no buildings and the only notable feature is a pier with a charter ship docked at it, and several Tyras Guard positioned there. Although the port makes it easier for players to access Tirannwn, King Tyras's men cannot use it to escape the Iorwerth Clan as they must remain to fight against their Dark Lord.
Port Tyras map
Port Tyras

Port Tyras, as seen from above.