hendo
Chieftain
Finally, I changed the fortification bonus from 25% per turn for two turns to 20% per turn for five turns. This means entrenched units gain a 100% bonus, easily overcoming the penalty in flatlands but without eliminating the ability to decimate units moving through flatlands. If you want more static lines with tougher defenders this is all you need to do, unlike a flat 'always on' defensive strength bonus this only applies to units that have had some time to prepare making hit & run tactics still a viable option against units that are moving. This also makes units significantly more resilient to ranged attacks if they are entrenched and makes it difficult to completely overpower a fortified unit flanking bonuses alone.
Under current rules, a fully fortified, full hp tank (base strength 50) defending against another HP tank, in neutral territory, with no modifiers other than tile bonuses, will have a strength of either 58.5 (clear tile), or 87.5 (rough tile). While I don't dispute that a defensive tile bonus should apply, the contrast between these two figures is a little jarring. The attacker almost break even with the defender in open ground, despite its fortification, and will come off much worse against the tank in rough territory, with the defender scoring a notable victory over the attacker, but the first scenario seems a little skewed in the favour of the attacker when perhaps it shouldn't be.
Under MJ's rules, the defending tank has a strength of either 85 (open), or 100 (rough). Here the defender will defend fairly well in open land, and will decimate the attacker in rough land. Bearing in mind that the defender is fortified, one would except the defending unit to be tough to destroy. Imo, the open territory bonus is a little high, perhaps just 0% (75 strength) is more balanced, or maybe 10% (80 strength).
Under Seven05's rules, the defending tank has a strength of either 88.5 (open), or 110 (rough). Combat results would be fairly similar to those seen under MJ's rules (with the defender doing a bit more damage), but the key difference really being how long it takes to fully fortify units, 5 turns, as opposed to the current 2 turns.
I personally like the idea of meshing the two ideas together, with MJ's new territory rules (but with 0% for open tiles instead of 20%), combined with Seven05's rule of taking longer to fortify BUT make it 10% per turn for 5 turns, so that the maximum fortification is unchanged, but it takes longer to build up. I wasn't as keen on the double HP idea

Just my thoughts and some numbers anyway, I might have got stuff wrong.