Tiles on to previous tiled solid wall

PVA the wall first and wait 40min to dry you can fill the obvious holes & depressions with adhesive pref rapid set, I would then use my 3m bar level or a 6foot level depending on wall size to check the level of the wall , it can be concurve or unlevel in the middle or at the edges. In this case I would mark the Areas with a pensil then generously spread the Rapid set where needed - then scrape the bar or level down or across the wall scraping off Excess rapid set adhesive leaving the wall level and ready to tile. Hope this Helps .