Eruv walls must total more than 50\% of the eruv border on each side, so that most of the expanse of the eruv is enclosed, either by an actual wall or by the form of a doorway (tzurat ha'petach), with no gate or gap over 10 amot (about 16' 8”, or 5 m) wide.
Note  In order to count as part of a border for a private domain, any doorway, gateway, archway, etc., must be intended to be a doorway, gateway, archway, etc.--and not there merely for decoration or function (as in supporting something above).