Labyren demo is fully playable on Steam Deck!
Making a Tower Defense game that is comfortably playable on a controller is a challenge in of itself, but if that's possible, then Steam Deck shouldn't be an exception. Most of the challenges arise in how to control selection and building placement.
Controller glyphs in some shape or form are always present when you are expected to take an action. For example here, in free selection mode, you can cycle through built towers or incoming enemies with the Left Stick, and with a standard Submit button (A on the deck) you can inspect their stats.
When building a new tower, helpful glyphs appear on the right side of the outline of the building, much like in the keyboard & mouse version. If buttons are accessible with the click of some button, that button's glyph appears on the button.
Hope this helps!