The tower has an on/off switch that will stop it from spawning mobs. It also has a panic button in case something goes wrong so you can seal the portal before a creeper can get loose. It also has a safety where if the glass block is broken sand will drop and block off the mobs. I added this after being blown up during testing. Unfortunately water will still flood out but you won't be dead. It's an easy repair, just go up the ladder and add more sand.
And the Colossal_Mob_Tower .schematic
|Colossal Mob Tower|
|The mobs come down these flow channels.|
|This switch will turn the entire tower on or off. It keeps mobs from spawning.|
|The panic switch. The ladder goes to where the sand can be reloaded if there is a mishap.|
|I used a half block but it didn't work so I went back to the glass block (not shown).|