OK, so in order: Either the starting range would have to be reduced massively (All the way down to 2), or the range bonus would have to be every 40 levels, because your system would allow it to hit literally any part of any map. Personally, I'd also get rid of the leader ability in exchange for a minor buff to base stats, but that's personal opinion. Definitely a cool idea though!
As far as range is considered, I purposely gave it high range, that's why its called a mortar because it can shoot at long ranges. plus it cant shoot at base 1 or 2 squares adjacent to it but can shoot at other areas of map. But yes range bonus can be kept at 25. Well then in place of leader ability the mortar can focus fire on 1 target in its range or something. Much like huli passes banana to Muli similarly all the mortars can focus fire. The leader system is optional
I know you want them to have extreme range, but my concern would be that Blood Demon Blade and/or Seelenreiser could be using to simply sit there and smash every single wave unopposed. It might be different if the "Spotter" was a separate, unique tower that is required in order to let the Mortars fire at long range, as then the individual mortars couldn't be abused all to hell. For me, it'd be something like: Mortar - Rare - Metropolis 4 Range, 5 sec, 40-50 Base Damage Mortar cannot hit enemies within two range of itself Ability - RELOAD! - Massively increases fire rate of all Mortars (Global Cooldown). "These cannons were once used to shell enemies during The Great Wars." Spotter - Unique - Metropolis 2 Range, 7 sec, 80-110 Base Damage +1 Range per 16 levels Allows Mortars to fire at targets he can see, regardless of their range. "They'll never see me coming."
Well mortars are Bad at hitting flying targets. Maybe this could be also a thing. And its also Quote hard to hit normal targets AS well because of the long time till impact. So maybe there are no direct Hits and there is only splash DMG.
Yeah we can also include it that mortar won't attack air targets. Just like Bear Hunter has debuff against air. what do you say.??? Cool this looks even better. Maybe it'll add the fun.