Xehh II,
To build a cathedral you need 6 churches. Cathedrals can only be built in citadels. You seem to meet all requirements to be able to build one.
Musketeers will be available through a port and armoury on castle level, then build a tradingpost on the fortress level and finally a gun factory on the citadel level. If my memory serves me right you can only get the muskets through trade with the Dutch.