Hello!
J1M told me to check this post out as he already knew me from before (in fact I joined the forum because of him, in a way) and knows I would know all about this coin!
So, the the coin is a Ceitil from Afonso V of Portugal (1438-1481), worth 1/5 of a "black real" (soon after the ceitis would become worth 1/6 of a real, a value which continued until their ultimate disappearance in the 1560s).
If you care, I can tell you that in the reference book for Ceitis, from Francisco Magro (whose title is actually just "Ceitis") it's the reference 2.1.4. and in terms of chronology this means it was likely minted between 1449 and 1457.
Here is a breakdown: Group 2 is "Tall walls with columns and no battlements, shield surrounded by crosses", subgroup 1 being "Walls with 6 interior columns, 5 battlements in each tower and shield of the 3rd type" and finally variant 4 being "sea with two loose waves".

Hopefully this satisfies all your need for information on the coin, and if not don't hesitate to ask for more!
Benny