Runes can be useful but your best bet is having very strong recon in combination with Death Coil. If you are human then of course use polymorph.

Is runes an effective way to stop late-game sappers if you don't want to build guard towers to protect your expansions?

I realize that usually saps don't play a large role in late games, but I was in a 3v3 where one guy was making tons of sappers while his partners were massing ogres to protect him and invade behind the sappers...

or they could use invisable instead if they just got slow blizz and invis then they save 2k

I use invisibilty on sappers when I'm human, I hardly ever bother researching poly unless it's a tree'd off type map or a game where poly becomes worth 200 mana a pop.

a) I'm orcs 99% of the time

b) the problem with using units to foil sappers is that you have to have your attention on the town, and be ready to click on a mage/dk and cast the spell when the saps come, and be quick about it (since the saps are pretty fast).

I really wanted something that could kind of automatically protect against them.