I read chapter 2.
Depedning how the rest of the book goes, but this chapter wont become Tower Defence without taking some liberties.
There is no actual fighting in this chapter, It just mostly keeps telling the changes Diabolus makes in Mansoul.
Defacing of Shaddais statues by Mr. No-Truth and Changing Major to Lord Lustings and Recorder (the consience) to Forget-Good.
"officers, governors, and magistrates. And these are the names of the chief of them: Mr. Incredulity, Mr. Haughty, Mr. Swearing, Mr. Whoring, Mr. Hard-Heart, Mr. Pitiless, Mr. Fury, Mr. No-Truth, Mr. Stand-to-Lies, Mr. False-Peace, Mr. Drunkenness, Mr. Cheating, Mr. Atheism-thirteen in all. Mr. Incredulity is the eldest, and Mr. Atheism the youngest of the company. "
Built three building:
"Hold of Defiance, because it was made to command the whole town, and to keep it from the knowledge of its ancient King. The second he called Midnight Hold, because it was built on purpose to keep Mansoul from the true knowledge of itself. The third was called Sweet-Sin Hold, because by that he fortified Mansoul against all desires of good."
Hold of Defiance governor appointed was Spite-God
governor of Midnight Hold was one Love-no-Light
the governor of the hold called Sweet-Sin Hold was one whose name was Love-Flesh
"that he should keep watch by day and by night at all the gates of the town, especially Ear-gate and Eye-gate; ‘for I hear of a design,’ quoth he, ‘a design to make us all traitors, and that Mansoul must be reduced to its first bondage again."
And then it mentions about how Diabolus prepares agasint Shaddais possible attack and how Diabolus keeps couple of (lying) speeches to people of Mansoul too long to copy-paste in here.
But in the end it mentons how Arms (weaponry) are given to townsfolk:
"‘1. My helmet, otherwise called an head-piece, is hope of doing well at last, what lives soever you live. This is that which they had who said, that they should have peace, though they walked in the wickedness of their heart, to add drunkenness to thirst. A piece of approved armour this is, and whoever has it, and can hold it, so long no arrow, dart, sword, or shield can hurt him. This, therefore, keep on, and thou wilt keep off many a blow, my Mansoul.
‘2. My breastplate is a breastplate of iron. I had it forged in mine own country, and all my soldiers are armed therewith. In plain language, it is a hard heart, a heart as hard as iron, and as much past feeling as a stone; the which if you get and keep, neither mercy shall win you, nor judgment fright you. This, therefore, is a piece of armour most necessary for all to put on that hate Shaddai, and that would fight against him under my banner.
‘3. My sword is a tongue that is set on fire of hell, and that can bend itself to speak evil of Shaddai, his Son, his ways, and people. Use this; it has been tried a thousand times twice told. Whoever hath it, keeps it, and makes that use of it as I would have him, can never be conquered by mine enemy.
‘4. My shield is unbelief, or calling into question the truth of the word, or all the sayings that speak of the judgment that Shaddai has appointed for wicked men. Use this shield: many attempts he has made upon it, and sometimes, it is true, it has been bruised; but they that have writ of the wars of Emmanuel against my servants, have testified that he could do no mighty work there because of their unbelief. Now, to handle this weapon of mine aright, it is not to believe things because they are true, of what sort or by whomsoever asserted. If he speaks of judgment, care not for it; if he speaks of mercy, care not for it; if he promises, if he swears that he would do to Mansoul, if it turns, no hurt, but good, regard not what is said, question the truth of all, for it is to wield the shield of unbelief aright, and as my servants ought and do; and he that cloth otherwise loves me not, nor do I count him but an enemy to me.
‘5. Another part or piece,‘said Diabolus, ‘of mine excellent armour is a dumb and prayerless spirit, a spirit that scorns to cry for mercy: wherefore be you, my Mansoul, sure that you make use of this. What! cry for quarter! Never do that, if you would be mine. I know you are stout men, and am sure that I have clad you with that which is armour of proof. Wherefore, to cry to Shaddai for mercy, let that be far from you. Besides all this, I have a maul, firebrands, arrows, and death, all good hand-weapons, and such as will do execution.’
To make it into a game, Two plans come to my mind.
The Arms part is obvious. Those shields and weapons can be somehow obtained to Diabolus side (a shop to buy them between levels?)
first plan is to use these buildings and other mentioned at beginning as bonuses that you can obtain.
For example it could be so that there is always normal and hard version of each level. If you complete either one, you get one star from each. If you complete them perfectly (ie. first level captain resistance not alarmed but sneak shot dead) you get another star. Or maybe even complete level in certain amount of turns.
Anyway, these stars could then be used between level to buy bonuses. And these bonuses could be like "Build Hold of Defiance - 2 percent bonus on attacks against.." or "appoint mister Atheism as governor - +5 defence for Shield of Unbelief".
Another way that comes to my mind is to take liberty from the story and make the conquering of town expand more than just one level.
First level being the chapter 1 and after that coming levels where there are fights to for example destroy statues of Shaddai. These could work so that there are buildings that keep producing more attackers each turn, and in order to complete the level, you need to destroy these buildings so that no more defenders come attacking you.
Those are my thoughts now from chapter 2