As a technology, walls are incredibly functional. A couple recent examples of border walls effectively keeping out invaders and/or terrorists are Israel and Hungry.

Israel: Israeli officials (including the head of the Shin Bet) quoted in the newspaper Maariv have said that in the areas where the barrier was complete, the number of hostile infiltrations has decreased to almost zero. Maariv also stated that Palestinian militants, including a senior member of Islamic Jihad, had confirmed that the barrier made it much harder to conduct attacks inside Israel. Since the completion of the fence in the area of Tulkarm and Qalqilyah in June 2003, there have been no successful attacks from those areas. All attacks were intercepted or the suicide bombers detonated prematurely.[51] In a March 23, 2008 interview, Palestinian Islamic Jihad leader Ramadan Shalah complained to the Qatari newspaper Al-Sharq that the separation barrier "limits the ability of the resistance to arrive deep within [Israeli territory] to carry out suicide bombing attacks, but the resistance has not surrendered or become helpless, and is looking for other ways to cope with the requirements of every stage" of the intifada. - wikipedia

Hungry: Hungary has slashed illegal immigration by over 99 per cent after rolling out a series of powerful border fences in response to the European migrant crisis.

https://www.rt.com/news/403738-hungary-border-fence-migrants/

Note: Walls are just the physical barrier that cost the attacker time and energy. They need to be backed up with effective and timely responses by humans (or their assignees) to take further action.