We decided that the player shouldn't be able to remove placed monsters to make the game harder. In our minds, it would be more interesting because now the player has to counter a given hero without making the dungeon too difficult for other heroes. So the player has to leave room for health potions, combining certain monsters and traps, and balancing those things to make the dungeon beatable for every kind of hero. We were afraid that without this the game would become trivial, but seeing the feedback from players, many of them are not fans of this mechanic.
So (if we decide to make a full version of this game) I'm thinking about this: adding the ability to remove monsters and traps, but it would cost some money and the price would increase every time an object is removed.