It’s important to pick the right hearthlings for the right jobs early on – for your footmen, use the hearthlings with high Body stats; and for the Clerics go with hearthlings who have high Mind and Spirit (ideally high Body too; but you’ll usually only get 2 of the 3 to be high… a 4/4/4 is good as an all-rounder; but my ideal Cleric is a 3 Mind/5 Spirit/6 Body). You can afford to go with lower Body scores if you plan to train your hearthlings more to get extra health that way; that’s easy enough to do with the Cleric if they have a mid to high Mind.
In Hard, you’ll want to power-level your defenders – get that footman patrolling on day 1, and give your Cleric-to-be a Herbalist setup ASAP. I would say that on Hard, the best starting items are a Practice Sword, an Herbalist’s Staff, and a Weaver’s Spindle. You might think that the Farmer’s Hoe is a better option; but you can survive on berries for a few days and it doesn’t take long to level your Carpenter enough to craft a hoe; but the Weaver can really boost your Footman’s early defence with even the most basic armour.
Make sure to get a wooden shield ASAP too, since those things are invaluable. In fact, make enough to supply your entire population with them – not only will it help level your Carpenter quickly, but it means that if you have to conscript your workers as emergency footmen then they’ll have a decent ability to outright avoid damage thanks to those shields.
As for that Herbalist, keep them constantly making Small Healing Tonics until they reach level one, and then start spamming Energy Tonics. The Energy Tonics give more experience, but more importantly they allow your hearthlings to move faster – which means your civilians can escape more easily, and your soldiers get into position faster.
And yeah, even with all this power-levelling in place, the biggest thing is to protect yourself from attacks so that you’re not caught being attacked from all sides. The easiest option is a simple dry moat, 2 blocks deep; although that looks ugly… unless you were already planning a wall. If that’s the case, you can use a neat trick: dig the dry moat where you want the wall to be, and when you’ve built the wall on top, dig the moat another two blocks deeper (or dig it out even deeper and wider if you want to add walls, floor, even a ceiling…) What you now have is a secret escape tunnel running under your walls, allowing you to move your soldiers between the gates if the enemies get into one of the gatehouses; and also allowing you to evacuate civilians to a bunker somewhere. In a worst-case scenario, the soldiers can stand guard in the tunnel to block the monsters from getting into the bunker.