About the defense, you need a lot of it so that you don't loss your units when king carlos raids it on daily basis,

About best defensive units in my view:

Armada

Submarine

Fleet

None

Mercenaries

Grenadier

Pirates

Sapper/Gun Slinger

 Bonnies

I had prepared this list by keeping in mind the units of carlos, since they are always mixed, fleet + armada + pirates & mercenaries. So we need to put units who have equal defense for each one of it or at least similar. 

I have put units from all the types who have similar number of defense against other units but i always go for Submarine & Grenadier. 

Do not forget you can always create a combination by using multiple units to meet the same numbers of defense for each attacking type.

Additionally this forum is for game suggestion and feedback's. The thing you asked is basically Game Discussion. So please review the small summary given after the heading of each forum so you can post your thread in correct section.

we request that everyone adds everything of there defense, 

as protecting your presid is better then losing your troops defending your haven alone,

defense works way better in numbers, the more you have the greater chances in defending

My choices go by what the players can make

Those that can make 

*Armada units i ask for 

Submarine s

*Fleet units

Man Of Wars

*Mercenaries

Grenadier s

Of course even pirate units are good for scrap troops

id say Bonnie's anything after the second page are mostly rum eaters and they just build up and eat your rum

once they start discovering mercs i ask for less pirate units and move up and start building more merc units

once they get enough mercs i allow them to start using there pirate units in prizes 

Do everything is in stage 

Pirates

Mercs

fleet

Armadas

Then gear units once you can discover them 

thanks was a lot of help