Probably worth noting that castle designs at the time basically screamed "this is a trap". Castles were designed with a courtyard inside of the first gate. Then there was a second gate on the other side of the courtyard. Attackers would be funneled into the courtyard, where they would be surrounded by archers on the tall palace walls. Notice this palace has several courtyards arranged in series, with gatehouses between each one:
Sima Yi's fear was that if he had his army enter the gate, it would be closed behind them and they would be trapped in the courtyard. It wasn't just a sarcastic "Hey come on in" signal as a Hail Mary. It was a very clear "My hidden archers are going to chew you up as soon as the gate closes behind you" signal.