Three ideas.
First of all, you could add more tile types as the game moves on to make the gameplay less repetitive as the game goes on.
Horseman has got to go. I suggest replacing him with a more customizable Ranger-type Character. The player could choose at level five if he wanted him to be a ranged or Melee character, and it would work out perfectly if there was a fourth weapon type, the player would be given a choice of a Bow, Crossbow, Musket, and Sword. As for slot 2, the only thing I suggest adding is an offhand dagger.
Bow: DPS is more in speed, excellent range
Crossbow: Balanced DPS, perfect Range
Musket: Slow speed high damage, lowest range
Offhand Dagger: adds damage, but little to no HP bonus.
Lastly, there could be some way to help your Heroes other than tiles. You could add in a Palisade Guardian style weapon that you can pull up and pop some people with. It could use Ranger weapons, and have a 10% nerf on Power to balance it, and to keep you working tiles.
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