And how do the dwarfs know about it? The only dwarfs there are the Legion of the Dead and I have never seen them use explosives. Further, the lore behind the game makes it clear that the dwarves are outnumbered massively. The only reason that they haven't been wiped out is that darkspawn aren't terribly bright except during Blights.
As for the Tower of Ishal, the archdemon is the one providing the brains for all the darkspawn. It seems to have sent scouts. When you arrive at Ostagar one of the guards mentions that they've found darkspawn in the lower levels.
The Dwarves at some point would want to reclaim the old Taigs of their empire and such...destroying them is not really something they would be too keen on...and causing instability in one part of a cavern system can have devastating effects elsewhere.
And the darkspawn could easily know about the tower via scouts...they may not have known the exact plan with a beacon and such, but using it to get behind the ferelden line is a sound idea