To many blocks not corectly aligned probably or jutting out from the edges etc.
Try to align them perfectly.
If you have lots of little blocks for detail, convert them to 'detai blocks' so they wont be rendered as veiwable portals.
Then in your compile options, reduce the tabs dealing with 'tessalation' towards the FPS range to reduce portal count, keep experimenting with these untill you get a balance for FPS and nice looking shade, not shadow.

If you have complex corners and intersections, you will also get a high portal count (refer to the manual regarding portals) but most of it can be scaled back with carefull level design.