As you are using maptool you can always just place VBL on the pillar. The players will see that it blocks their vision and so will realise its a pillar rather than a hole.

Equally, giving the whole pillar a drop shadow will make it clear that it comes out towards the viewer rather than into the page and it will allow you to re-use your pillar. If you're drawing it in place then a nice long directional shadow as Ascension suggested would certainly make it clear that the feature was a pillar.

Otherwise, these look like good first maps for a dungeon crawl. Keep posting them up!