Screen is currently at F2, where a few example bridges have been built. Notable areas are marked as zones ("i" to find them).

Zone 11 is an area of branches touched by two bridges, but all attempts to build there are blocked with the "no access" pink cross; in both cases, dwarfs would need to use a bridge to step across twigs, which doesn't work.

Zone 12 is reachable via bridges/branches and constructions there are built without issue. Bridges over open space establish walking connections.

Zone 14 on the level below shows the outline of the bridge immediately above. Checking tile identities with "k" shows "outside" under all tiles where the bridge coexists with branches/twigs, "inside" where the bridge is on open space.

Whether a bridge is on open space or a tree feature must be checked via "k" on the individual tile; only the bridge is displayed.