The most simple solution is this: 4e allows for exception based design, and sometimes very persnickety exceptions, so just say "You can't teleport here. The end."
A more helpful solution: a ritual, ward, whathaveyou, that limits line of effect. It's like being blind, except that you are power-blind. This might be a power that continues throughout the entire area; it might even have implications for non-teleportation magic, thus resulting in limiting ranges of various powers. (Yes Mr. Wizard; your magic missile fizzles after two squares.)
Also, as suggested above: assume people will teleport, so build safeguards. For instance, there's 5-10 feet missing between the outer-most wall and the interior floor. So if someone teleported from the outside in, they'd end up falling through the gap. Another solution is a Teleportation Funnel; if someone uses teleportation inside the Fortress, then they are re-directed to a pre-designated location (say, a cell, a monster's cave, etc).