The_Universe said:
You don't know how the tower gets its energy, so you'll probably have to depend on effects that - though temporary - are almost always effective. Stick with reliable.
Start with legend lore on the tower - you can see it from outside the city, so the spell takes a mere 1d4x10 minutes. I'd expect things like
"the only way in is..."
"only wizards and their advisors may enter"
"there's a password"
"the magical defenses of the tower are fuelled by"
"the one weakness of the tower is..."
"they say that the attackers have secret agents who are working to bring down the tower"
Run through some of that information with contact other plane. At this sort of level, a DC 16 int check (for talking to a greater diety) should be easy.
"If we were to kidnap someone who has access to the tower, who is currently in the city, what is the name of the one who would be the easiest to access?"
"Will he be accessable during the day, or night?"
"Morning or afternoon?"
"Street name?"
"House number?"
Then, run with the lore that I got, handing it to a cleric using commune.
"If I destroy X, will that take down the tower?"
"Are there regular patrols within the tower?"
"Is there more than one patrol at a time?"
"More than two?"
"More than three?" etc.
"Are the patrols more regular than hourly?"
"Half hourly?"
"..."
"Constant?"
"Do the people in the tower recognise each other by sight?"
"Does the tower have someone in it that can detect invisible creatures 24/7?"
"Will my party be likely to prevail in an encounter with a single patrol?"
"With two patrols?"
At the same time, the sympathisers within the city will be gathering the same sort of information.
In short - my plan will be ENTIRELY dependant upon what YOU the DM have set up. I will gather very detailed information about the workings of the tower. I will formulate a plan based on that information. I will act based on that information.
Can you tell I play shadowrun?