entrenched troops, artillery and fortifications.

once a qing player sent his entire army to try and take me, (i was nepal) and all his troops died and he had like 500k losses.

eventually i sent a peace treaty saying i would take all of his economy and resources and he accepted lol

More specific: Build a fort, put 5 tank divisions each with 200, 4 Arty each with 50, Infantry varying based on manpower. Submarines surrounding it if it is a coastal city. Did this as Princedom of Duklja. Made Ottoman empire ragequit.

Also nice to have anti air, just in case