I have an idea: if the map would be a prison break map. That would support the theme of 1 spawn/2 exits. But, the spawn would also be a prison tower, that tries to stop the creeps from escaping. The tower could have the ability to construct, just like Mr. Iron, but instead of becoming totally useless for the time of constructing, the Prison Tower will summon guards to shoot from it. DMG could be reduced to a quarter of original and attack speed doubled, since there are 2-3 guards shooting. This way the Prison Tower would not have to have the ability to scale with attspeed. Also, it would be unable to get sold. Also, the placement could be out of Mr. Irons range when added those 1 range aura items. If made main carry, the two constructing towers cannot benefit from each other.
Name: Prison Tower
Rarity: (none, since being always there)
DMG: 40-50, increases slowly per level
Attrange: 2-3, depend on balancing
Abilities: Construct, just like Mr. Iron but not scaling with attspeed, 60s, Summon Guards, tower attacks at reduced damage (1/4) and increased speed (1.5-2x, depend on balancing), if still room for more: Sound the Alarm!, makes all nearby towers (on range range of 3-4) to attack faster for some time, like 10s.
Description: No prisoner has escaped AND survived from this prison.
Health can mean the wizard's mental health as put in charge of anti-escape system developing. As creeps get through, wizard loses health and every tower gets +1(0)% base dmg bonus/crit & dmg as the wizard is getting insane (towers get more bloodthirsty), but suffers a -1(0)% attspeed, since the insanity causes them to think longer before taking a shot.
I think this sort of idea changes the entire game thematically and would be hard to implement.
But I like the overall premise if you stuck to this:
Description: No prisoner has escaped AND survived when the Watchmen watch.
Skill (Passive): Sound the Alarm! When a creep comes in range, a klaxon sounds, making all nearby towers attack faster for 2 seconds.
Skill (Active): 60s, Summon Watchmen, tower attacks at increased damage (x2) and increased speed (x2) for 2 seconds.
> Basically, the active ability should be something you want to use. At the moment, all you're doing is sort of reverting the damage to normal (increased damage + decreased attack speed = average/same damage). You also want some sort of passive effect that occurs when the tower isn't doing anything - I modified your second ability to become the passive.
You could change the passive to be a slow (like, creeps tip-toe past the watchtower, hoping not to get caught), which would also be useful.
Anyway, lol. That's my take on your idea.