Frankly, I don't understand the question. What does a permanence of a fort change? I use forts in EB as strongholds on my border since long and will hopefully be able to do so in EB II too. Or will this not be possible? Just put the 20 men or so rest of a cheap unit (after a fight) in a non-permanent fort or use a cheap total unit - it will also represent the necessary garrison without which no castle or fort ever would have made sense.
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