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We'll start off small weakest fort to smallest fort.

Thieves den Rating from (1-3): 2.3
Navy and EITC found there

Navy
Cadet lv 5 to 5
Guard lv 5 to 8
Seargent lv 8 to 10
Veteran 15 to 18
Officer lv 20 to 23

EITC
Thug lv 8 to 10

Fort Dundee Rating 2.4

Navy
Seargent lv 9 to 12
Veteran lv 15 to 18
Officer lv 21 to 23

EITC
lv 0 to 0

Fort Charles Rating 2.8

Navy
Cadet lv 2 to 5
Guard lv 5 to 8
Seargent lv 8 to 12
Veteran lv 15 to 18

EITC
Grunt lv 12 to 15
Mercenary lv 26 to 30

Kingshead Rating 3.0

Navy
Seargent lv 9 to 13
Veteran lv 15 to 18
Officer lv 20 to 30

EITC
Grunt lv 15 to 18
Hired gun lv 20 to 22
Mercenary lv 26 to 30
Assassin lv 30 to 35

Black Pearl Island Rating 10
Navy
Two guards at beginning lv ? to ?
Officer lv 28 to 39

This is the complete guide to navy forts.