I'm not really seeing a win condition for staying in the Tower. Etelka won't come here since her spell-casting will destroy it. Similarly, why would she go to where a gunman could snipe her from a vantage point?
In contrast, it seems like she's here to punish the mutants who betrayed the cult. She has a clear goal here, and we can use it to predict what she'll do next. If we know where she'll go, we know where she'll be vulnerable to a bullet to the head.
The only way to win this battle is to kill the witch, and the mines is where we have the best chance of doing that.
In contrast, what does staying in the tower net us? A semi-defensible position, less so than the mine, that's completely divorced from our win condition (Kill the Witch, save the mutants) and lacking in manpower to make full use of it. Etelka can just mire us in Beastmen while she completes her actual goal and escape or turn her attention on us after we're exhausted, and we'll be an apothecary short with a lot of dead mutants whose hopes we ignited only for that to be snuffed out just as quickly.
That more than anything, will break Markus' spirit and a defensive position is useless if the men behind them have their morale damaged.