Titanfall
Titanfall is a multiplayer first-person shooter video game, developed by Respawn Entertainment and published by Electronic Arts. It runs on a heavily modified version of the 
Source Engine based off 
Portal 2 engine branch[1], with a custom version of BSP, a custom version of the VPK package format, and an improved rendering engine. The sequels and spinoffs, 
Titanfall 2 and 
Apex Legends, both use this same engine.
Titanfall restricts typical Source engine features such as the developer console and server browser, but these can be re-enabled with offline mods.
As of 2021, Titanfall is no longer available and has been delisted from Steam as the game servers has suffered from DDoS attack and the game was left unpatched with many known security vulnerabilities. The game later received review bombing on Steam shortly before and after it got delisted.
Limitations
Important:Servers are down since 2021 according to several reviews on Steam.References
1. According to the sequel Titanfall 2's steam.inf or gameinfo.txt containing leftovers appid (620) for Portal 2[confirm]