Awhile back a modder going by Kallale created a mod that changed Rook's towers so that they would not disappear like they normally do if you reach the tower limit or if X time has passed. You can read about that original mod here: http://forums.demigodthegame.com/369904/
Anyway, the links he created in that thread no longer work (eg you can't download his mod anymore), so I've created a new mod that does essentially the same thing.
Please note - I'm describing what files were modified below. You don't have to code this yourself. You can just download either of the installers at the bottom of the page and you are all set. Just see the notes in case you want to create your own mod!
Here are the files that needed to be altered and can all be found in units\heroes\HRook
If you edit the HRook_Abilities file, you can change the cooldown, energycost (mana), range, etc. You can find all of those settings and where to change them if you look for Power of the Tower I in that file. Please note, you will want to make the same changes to Power of the Tower II and so on.
The files starting with HRookTowerOfLight_script (2,3, and 4) all were modified to keep the towers from being destroyed after x seconds. Just scroll to the bottom of the files and see my note. FYI - you can comment out lua code with # signs (and that's all I did here, so you can still see the original code intact).
Version 1 (http://www.box.net/shared/s3d0dbk3ua)- You can build up to 99,999 towers and no towers are destroyed over time (they can still take damage of course), but all of the regular cooldowns, costs, are in place
Version 2 (http://www.box.net/shared/7pexv8m841)- Same as version one, but I've reduced the cooldown time to 1 second, so you can spam towers faster.
To install, just download the executable of your choice and run it. Then, be sure to enable it in the mod manger. You should only have 1 of these versions enabled in the mod manager at any time.
And a final note - this mod will work fine in SP. If you want to use it in MP, ALL users must have the same version of the mod or the game will desync. Have fun!