Including Steam itself?

I.e. you don't have to run Steam to run DOTT, if you bought it from there.

Steam can be run in offline mode.

Unless it decides that it can't and then you either go online or can't play.

Friendly DRM systems like the AppStore or Steam might seem a good idea until one gets burned by them.

I should know, still haven't learned my lesson.

Many old games like this that are optionally available through Steam don't actually integrate the DRM, so you can run them outside of Steam as well.

The old UFO/XCOM games for example are sold through steam, but ship as a DOSBOX.