Swordmaster, I think there is a bug that occurs sometimes if your vehicles get repaired and the enemy no longer recognizes that unit and doesn't target it. I play on v. 1.07. So that would be why your vehicles on level 5 weren't attacked. I can't remember if the vehicle needed to be getting fired at (shot in air) prior to the pickup to repair for the bug to happen. I just play try to not use those vehicles because the game isn't as fun.
Also the Fremen attacks can be a viable option, idk if the game glitches but sometimes the fremen attack and the enemy doesn't return fire. And if you don't watch them take down the buildings they are attacking then the building doesn't repair as fast. The problem to encounter, particularly on the last level is if your fremen divide up attacks on the palaces because you need all your fremen concentrated on one palace. I believe I had 9 rounds of Fremen and it was enough to take down the palace, but only if I wasn't watching the building. otherwise they could outrepair my attacks and I couldn't take down the building.
I also can't remember if there is a limit to the number of fremen you can have on the battlefield. Same with Saboteurs. Which I thought was 2 Saboteurs were the max, and I wanted to say 3 sets of Fremen but I just played the last level the other day and had 9 sets before I finished off the last palace. And it could be that 9 is the max, but since I usually have a minimum of 3 palaces going on that level that I remember that there is 3 overall sets of fremen from each palace. Anyways, I'll come back when I test that out actually. Just some food for thought.
Side note, When I was kid I loved the Death Hand so much that I played the last level by building 7 palaces and only used that to take down the bases. And then I ran out of money bc I didn't really understand the importance of harvesting and I got outharvested. The version I also played then was the one that costs money to repair the palaces, which is frankly not worth it as it was super expensive. Im pretty sure I tried to keep them pretty well repaired. The other thing that I recall about that version was that it seemed like the computer RARELY rebuilt buildings. So you took down that Sardaurkar palace once and you were good. In fact I remember when we got (ithink its a windows 95 version?) the game on a cd and we played and they were rebuilding buildings and it was so much harder. Especiallly that last level bc I couldn't build a palace at the bottome fire a shot and listen to hear if I got the palace. I now had to aim for the corner and then get the palace which generally meant I needed to endure a couple missiles and so forth. Anyways, good times.
