I've got a great tower design for the babel servers- its fast to build and darn near indestructible.

you don't just build stairs, you build a big structure. rather than waste alot of time building a solid structure, you build a quick frame. you put the stairs inside. the structure is very blocky- the design is based on the empire state building's structural steel frame. it's very strong and difficult to destroy due to its sheer size and number of beams that require to be destroyed. Early builds was just a staircase with a few extra legs on it.
Like this:

some inept teammates screwed up my stairs real bad, so I had to double back a little and built a new stair above the old one. and they screwed it up again... *sigh* ....idiots.
may latest variant of this design requires you build the frame first, then the stairs. the stairs is fully enclosed by the frame, so long as no one's a griefer it is quite unlikely the enemy can break it.
some downsides:
the fairly open structure has zero ballistic protection.
it still requires alot of blocks. build it near the command post if possible.
if you don't give it floors you'll often find yourself falling to death.
it's not the prettiest thing around... well if you felt like it you can give it a pretty facade, just like the empire state building.
some design considerations:
the structure will be at least 60 blocks long, make sure you have the space
make sure there's no inept players on your side, they can seriously ruin the build
its very resistant to enemy attack, however is not indestructible.
the more "legs" you give it the better.