In tower defense games, enemies advance toward your base in waves, and you place towers and traps in their path. Each tower attacks differently, and the key is finding the best combination and placement as the waves mount. Simple rules and deep strategy make this genre incredibly addictive.
Yes, it's one of the most straightforward genres: the rules are clear right away, and the difficulty increases gradually. Kingdom Rush and Plants vs. Zombies are good places to start.
Any of these games can be downloaded via torrent, repack, portable version, or GOG release.