Airlock
Side View:
<PRE>
  Z1   XXXXXRRR___L     
  Z0   D_____/XXXXX
</PRE>

D = Raising drawbridge  <BR>
/ = Ramp<BR>
_ = Floor <BR>
R = Retracting bridge<BR>
X = Solid Rock or Constructed Wall<BR>
L = Lever

Construct from the left to the right. Build and attach the Raising drawbridge to a lever. The build and attach the retracting one.