Overall a promising map, however I definitely think you should do something about the portals. Currently, they teleport you in the middle of one of the four sections; in the middle of a gameplay area, which means a zombie could randomly appear in front of a human. Portals in general should be placed carefully in order not to bring down the quality of the gameplay. I recommend removing the portals and just making a water ladder to the top and making a gap in the ground. I know the bottom section is supposed to be hidden from the top, so making a gap for the water ladder would reveal the bottom. However, in my opinion, it wouldn't hurt the map at all if you just removed the fall-through blocks entirely as well. I think it would look pretty cool, too.
This map gameplay wise looks like it would mostly be fit for rotation. The portals however teleport you to the middle of each section. Maybe the spawn point could be put higher so a human would have time to react to a zombie in the way. A water elevator seems a little boring to me and they have had problems on other maps. 2block jumps would flow better on the corners but that's not a big deal.