Well, the tower is set in an exact amount at BT.
But after re-checking, I think it's not that complicated.
It should be possible just by doing this (in this sample, I will only allow one tower to be chosen) :
1. Make both towers (the real tower and the tower ruin) at OE
2. Make a unit that can be bought with the info of the real tower and give that unit as a buy able unit in the ruin.
3. Create a trigger (sorta like this - Leaks Ignored) :
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Events
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Unit - An Unit Dies
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Conditions
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Unit-Type of Triggering Unit equals to The Real Tower
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Actions
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Unit - Create 1 Tower Ruin for Neutral Passive at Position of Dying Unit facing 270.00 degrees
Just add your real towers to the condition and use Or comparison
4. Then another one for the making :
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Events
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Unit - An Unit bought from shop (forgot the real event name)
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Conditions
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Unit-Type of Sold Unit equals to Real Tower (Dummy)
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Actions
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Unit - Replace Selling Unit with The Real Tower with max stat
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Unit - Change Ownership of Last Replaced Unit to Triggering Player (I think I'm not mistaken)
Some event, condition and action aren't correct (possibly) because its pseucode. But I'm sure you get the main idea now.
And that's what I can think in mind, also the "ruin" has Invulnerable, since in BT tower are a good asset (but not on late game when your tanks own towers in like 1 minute)