At some point you're going to free a slime who opens up a museum, but it won't have anything in it. Just a bunch of blank pedestals. It's meant to house statues of enemies. I think it's if you capture 30 of one enemy, you get a bronze statue. After 50 you get a silver one, and ultimately can get gold. I do not think it makes any difference at all in terms of the "level" of the statue.
However, getting the statue does have one good effect - you can then hire that particular enemy to join your tank army as an ally. Since all your tank allies have different abilities/strengths/speeds, it's good to experiment. Some of the enemies are highly useful - they can go cause havoc in the enemy's tank, act as a gaurd for yours, or fire themselves as ammo. Others might carry ammo for you.
I haven't played in a while, but there's this robot enemy you can find later on, and I think I have him in my army. Dude is brutal. Super fast and strong, he goes over to the enemy tank and kills the enemy commander's allies repeatedly.