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Well, you could spawn more bosses who would heavily encourage the use of these towers. The AI could make them always prioritize a tower first over chasing you.

There could be optional treasure caches where you rob the evil wizard's coffers to get more tower guys. They spawn on the map, and it's a handful of strong guys defending them. They drop coins.

It gives the player the choice of hunting down the obelisks or building up their defenses. It seems the longer the game goes on, the bigger the waves become? So there's a reason to try to finish things quickly.

The towers could patrol an area or be set to follow the player, or chase down and engage enemies within their range.

IE, you drop a knight, and he'll step into the melee to scrap with some imps. Perhaps his special ability enables him to body block people, so up to 5 imps are stuck on him and can't progress.

Then you're looking at an army simulator where you can respawn your guys when they die.

That's a genius idea, by the way. I love the fact that the coins are refunded when your guys die. It makes the tower system not frustrating at all, and you're free to create strategies. To plan. To make mistakes.

And in the future, you could make a boss who swallows towers when he or she defeats them, so that the coin disappears until the boss is killed. It would keep the player from just repeatedly using them over and over again.

There's also the idea of adding in synergies.

Like if firing an arrow at a knight meant you could bounce it off his shield, and then it'll ricochet into a different direction. Firing an arrow at a wizard's fireball will create a flaming, exploding arrow.

The player gets some tangible bonuses for using the towers and is encouraged to create and use them.

And then you could have another option that lets you sacrifice coins for EXP. So if someone wants to, they could get more personal power and forego towers. It creates an alternate route and play style. 

Likewise, if you had a monster tamer character or mechanic. If you could drop coins and non-boss enemies walk over them, then maybe they become your towers.

The monsters could have their own advantages that put them above usual towers, but maybe the coins don't come back when the monster dies. Possibly it doesn't need a drawback at all? Potentially, it's better as a character class option, and that class has no access to normal towers.

You could also have upgrading towers as an option. The tower gains EXP just like you do, and they'll walk over and collect points. Then they grow stronger.

If you made a cleric or paladin main character, they could have more interactions with the towers via buffing and healing them. Some of their upgrades could focus on improving allies and making them harder to kill and thus ensuring they can survive long enough to upgrade.