your right to hold the enemy at the bottle knecks but if u dont destroy them at source they'll keep coming!

i'm with these guys, destroy the war machine (the cities) & if u cant hold them destroy them & give them to an ally

if u cant storm a city u can do damage with artillery & assisins

blockade ports cut off the income, this may mean building up a navy big enough to destroy enemy ships first, dominate the seas

if u are fealin dirty u can always recruit spies in a plague infected city & send them to infect an enemy city