Speak to King Lathas after you have completed Breaking to Protect. He will ask you to go ahead building the port, and that the plan for it has been sent to the RPDT HQ. Go there and ask for them, with the Kings seal. However, the architect has suddenly dissapeared, and therefore the plans have not been sent. Talk to the king and he will say he used to live in Seers' Village. Take a spade, and search all the graves. The grave of the architect is different for all players. Searching the right one will yield some plans. If you do not have 15 Prayer, you will not find the plans.