You can't reach there by ships until you have high tier ships which you can get after world is round event from naval dry docks etc (Carrack, Caravel etc) and via land you have to go through:
When playing as a catholic faction if you want to capture these regions then it's better to send around 4-5 priests in advance to those regions (to get a easy control over the city after sacking them) before sending an army containing a general of high loyalty. One could also hire African mercenaries if you want to reach there early by only cavalry or only general.

Arguin is not really worth it unless you want to suck all the resources which will come in handy in the later part of the game and a chivalrous general will also help in faster population rise which in turns upgrades your settlement bringing in more income.